Theoretical Studies on the Role of Guest in α-CL-20/Guest Crystals

The contradiction between energy and safety of explosives is better balanced by the host–guest inclusion strategy. To deeply analyze the role of small guest molecules in the host–guest system, we first investigated the intermolecular contacts of host and guest molecules through Hirshfeld surfaces, 2-D fingerprint plots and electrostatic interaction energy. We then examined the strength and nature of the intermolecular interactions between CL-20 and various small molecules in detail, using state-of-the-art quantum chemistry calculations and elaborate wavefunction analyses. Finally, we studied the effect of the small molecules on the properties of CL-20, using density functional theory (DFT). The results showed that the spatial arrangement of host and guest molecules and the interaction between host and guest molecules, such as repulsion or attraction, may depend on the properties of the guest molecules, such as polarity, oxidation, hydrogen content, etc. The insertion of H2O2, H2O, N2O, and CO2 had significant influence on the electrostatic potential (ESP), van der Waals (vdW) potential and chemical bonding of CL-20. The intermolecular interactions, electric density and crystal orbital Hamilton population (COHP) clarified and quantified the stabilization effect of different small molecules on CL-20. The insertion of the guest molecules improved the stability of CL-20 to different extents, of which H2O2 worked best.


Introduction
As the most widely investigated high energetic compound, hexanitrohexaazaisowurtzitane (CL-20, with typical polymorphs of α-, β-, γand ε-) has the highest energy density [1], but it is still not widely in service due to its high sensitivity [2], phase transformations [1] and high cost. Host-guest inclusion strategy is an effective method to significantly alleviate the contradiction between high energy and low sensitivity. The development of host-guest compound explosives can solve the problems of laboriousness and risk of developing new energetic materials [3][4][5][6][7]. Bennion et al. [6] incorporated one solvate hydrogen peroxide (H 2 O 2 ) molecule into the crystal system of anhydrous ε-CL-20, and obtained the CL-20 hydrogen peroxide solvate (CL-20/H 2 O 2 ) for the first time. It had high crystallographic density (2.03 g·cm −3 ), high predicted detonation velocity/pressure, performed better than ε-CL-20, and had a sensitivity similar to that of ε-CL-20. Xu et al. [7] incorporated oxidizing gas molecules (N 2 O, CO 2 ) into ε-CL-20 to obtain the CL-20/N 2 O and CL-20/CO 2 complexes. CL-20/N 2 O exhibited a surprisingly high crystallographic density (2.038 g·cm −3 at 298 K), more thermal stability, better predicted detonation properties and lower sensitivity compared with ε-CL-20. The guest-accessible volume in α-CL-20, without being occupied by water, revealed sufficient void space to encompass some solvent molecules such as two H 2 O 2 , CO 2 and N 2 O. Little deformation of the lattice parameters was changed after removing the water under heating/vacuum from α-CL-20 [8]. Therefore, the crystal structures of (a) CL-20, (b) CL-20/H 2 O 2 , (c) CL-20/CO 2 , (d) CL-20/N 2 O, and (e) CL-20/H 2 O remained isostructural to the hydrated α-CL-20 [6,7]. A chemical diagram and molecular structure of etc.) of the host high-energy explosive CL-20. The practical application of the host-guest explosive is always inseparable from the interaction between host and guest molecules in the system. A study of the intermolecular interaction is crucial for understanding the behavior of the complex in the actual environment and facilitating its application in practice. Meanwhile, the intermolecular interaction is the central scientific issue of energetic cocrystals [11][12][13]. Therefore, systematic studies on the comparison of interactions between the host-guest energetic complexes constructed by embedding different small molecules into the crystal lattice cavity of α-CL-20, are necessary. Further research devoted to summarizing the influence of guest molecules on the performance of α-CL-20 in order to explore more host-guest energetic complexes, is necessary.
In this paper, we will explore the potential interaction between the host and guest molecules and summarize the influence of guest molecules on the host explosive. Specifically, Hirshfeld surfaces and 2-D fingerprint plots show the intermolecular contacts. First, we examine how the electrostatic interaction energy shows the strength of the intermolecular contacts. Then, we employ ESP and vdW potential to intuitively describe the electrostatic and vdW interaction characteristics of the host and guest molecules. This analysis provides us with a general understanding of the basic character of the intermolecular interaction of this species. After that, we carefully examine the COHP analysis, charge density and difference charge density of host-guest complexes; the composition of each will be useful for detailed chemical bonding analysis. This part of the research will help us grasp the chemical bonding variation of CL-20 by embedding different small molecules. Finally, we summarize which guest type has dominant influence on the host CL-20.  In this paper, we will explore the potential interaction between the host and guest molecules and summarize the influence of guest molecules on the host explosive. Specifically, Hirshfeld surfaces and 2-D fingerprint plots show the intermolecular contacts. First, we examine how the electrostatic interaction energy shows the strength of the intermolecular contacts. Then, we employ ESP and vdW potential to intuitively describe the electrostatic and vdW interaction characteristics of the host and guest molecules. This analysis provides us with a general understanding of the basic character of the intermolecular interaction of this species. After that, we carefully examine the COHP analysis, charge density and difference charge density of host-guest complexes; the composition of each will be useful for detailed chemical bonding analysis. This part of the research will help us grasp the chemical bonding variation of CL-20 by embedding different small molecules. Finally, we summarize which guest type has dominant influence on the host CL-20.

Computational Methods
The Gaussian 16 (A.03) [14] program was implemented to analyze geometry optimizations and frequency using the uB97XD exchange correlation function [15] in conjunction with the def2-TZVP basis set [16]. The process of geometry optimization is the process of obtaining reasonable structure. In order to obtain a reliable WFN file, it was converted from CIF file format. Then, Multiwfn 3.7 code, developed by one of the authors of this paper in [17], was performed to analyze the ESP, vdW potential, non-covalent interaction (NCI) map, contour map of electrostatic potential, electrostatic interaction energy, IRI and isosurface map of electron density on the basis of the optimized geometry. Visual Molecular Dynamics (VMD) software [18] was rendered to analyze isosurface maps of various real space functions based on the files exported by Multiwfn.
A freely available software, CrystalExplorer [19], was not only applied to visualize ab initio molecular ESPs mapped on Hirshfeld surfaces [20] or isosurfaces of the electron density, but was also used to calculate quantum-mechanical properties of molecules [21]. In the region of the hydrogen bonds, the isosurfaces overlap significantly, whereas Hirshfeld surfaces touch, and quite clearly demonstrate the way in which complementary electropositive (blue) and electronegative (red) regions of adjacent molecules come into contact in such an interaction [22]. The intermolecular interactions in crystals can be directly observed by electrostatic potentials mapped on Hirshfeld surfaces. The intermolecular interactions can be analyzed quantitatively and qualitatively by two-dimensional mapping [23,24] in a convenient color plot. The intermolecular contacts were explored by the points on the surface which were defined by the distances to the nearest atoms outside, de, and inside, di [25]. During the theoretical investigations by CrystalExplorer, the chosen part of crystal structures was automatically selected by CrystalExplorer software from the CIF files. The chosen part of crystal structures was periodic for the crystal structures.
Bader's QTAIM [26] method and DFT calculations with Critic2 [25] were used to investigate the intra-and inter-molecular interaction strength. The core and valence electron Gray, blue, red, and white spheres stand for carbon, nitrogen, oxygen and hydrogen atoms, respectively.

Computational Methods
The Gaussian 16 (A.03) [14] program was implemented to analyze geometry optimizations and frequency using the uB97XD exchange correlation function [15] in conjunction with the def2-TZVP basis set [16]. The process of geometry optimization is the process of obtaining reasonable structure. In order to obtain a reliable WFN file, it was converted from CIF file format. Then, Multiwfn 3.7 code, developed by one of the authors of this paper in [17], was performed to analyze the ESP, vdW potential, non-covalent interaction (NCI) map, contour map of electrostatic potential, electrostatic interaction energy, IRI and isosurface map of electron density on the basis of the optimized geometry. Visual Molecular Dynamics (VMD) software [18] was rendered to analyze isosurface maps of various real space functions based on the files exported by Multiwfn.
A freely available software, CrystalExplorer [19], was not only applied to visualize ab initio molecular ESPs mapped on Hirshfeld surfaces [20] or isosurfaces of the electron density, but was also used to calculate quantum-mechanical properties of molecules [21]. In the region of the hydrogen bonds, the isosurfaces overlap significantly, whereas Hirshfeld surfaces touch, and quite clearly demonstrate the way in which complementary electropositive (blue) and electronegative (red) regions of adjacent molecules come into contact in such an interaction [22]. The intermolecular interactions in crystals can be directly observed by electrostatic potentials mapped on Hirshfeld surfaces. The intermolecular interactions can be analyzed quantitatively and qualitatively by two-dimensional mapping [23,24] in a convenient color plot. The intermolecular contacts were explored by the points on the surface which were defined by the distances to the nearest atoms outside, d e , and inside, d i [25]. During the theoretical investigations by CrystalExplorer, the chosen part of crystal structures was automatically selected by CrystalExplorer software from the CIF files. The chosen part of crystal structures was periodic for the crystal structures.
Bader's QTAIM [26] method and DFT calculations with Critic2 [25] were used to investigate the intra-and inter-molecular interaction strength. The core and valence electron densities and difference charge density of each crystal were obtained from DFT calculations implemented by VASP [27]. During the theoretical investigations by VASP, the chosen part of crystal structures was selected by the nearest two neighbors. Furthermore, although LOBSTER was originally designed with interfaces to handle only wavefunctions from  [27], while using LOBSTER, the crystal orbital Hamilton population (COHP) analysis was demonstrated and reported in [28]. LOBSTER is a multiplatform tool that is written in object-oriented C ++ and parallelized using OpenMP. It employs Boost libraries [29] in addition to the highly efficient Eigen library [30].

The Intermolecular Contacts of Host and Guest Molecules
Usually, two interacting molecules stack in a special direction due to electrostatic attraction. The magnitude of the electrostatic attraction depends on the closer contact of the positive and negative ESPs [30,31]. From the surface minima and maxima sites (shown in Figure 3), the outermost distribution are minima of CL-20. The minima sites are evenly distributed in the extension space corresponding to O atoms of N-NO 2 fragments. This demonstrates that CL-20 more easily forms H-bonds with other molecules at the minima sites. Therefore, the distribution of guest molecules is affected by the ESP-mapped molecular vdW surface. When there are no hydrogen atoms in the guest molecules, such as N 2 O and CO 2 Figure 3.
To obtain a better understanding of the host-guest driven inclusion behavior between CL-20 and guest molecules, the intermolecular interactions [4,32] of single crystals were studied by freeware of Hirshfeld surfaces, as shown in Figure 4. In the Hirshfeld surface analysis, the red and blue areas represent the probability of close and far contact with external molecules, respectively. The red regions arranged in the oxygen and hydrogen atoms are shown in Hirshfeld surfaces. This implies the main intermolecular interactions contacts of CL-20 are focused on O and H atoms. The reaction sites of the H atom are consistent with the conclusion of Figure 3. The number of red regions (i.e., close contacts) is the same for the CL-20 fragment of the five substances. However, the sites of the red regions for the host-guest complex are distinctly different than for CL-20. The red regions around the guest molecules are much more obvious than around the host. The intensity of the red areas for guests decreases in this order: H 2 O 2 , H 2 O, N 2 O, and lastly, CO 2 . This demonstrates that CL-20 is much more stable after incorporating guest molecules. This conclusion is consistent with the finding that CL-20/N 2 O has higher thermal stability and lower impact sensitivity than CL-20 [32]. The d norm Hirshfeld surfaces for CL-20, CL-20/N 2 O, CL-20/H 2 O, and CL-20/H 2 O 2 resemble a whole shape. However, it is quite different for CL-20/CO 2 . The Hirshfeld surface is significantly different, in that it divides into two parts. The sites of the red regions for the CL-20 fragment are obviously biased to where the CO 2 molecule is located. The different results may be due to the different polarity of the guest molecules.
The 2-D fingerprint plot directly demonstrates the intermolecular interactions of internal and external distances of atoms from the surface. It is possible to show the range of structures by the changes in the fingerprint plots while adding the guest into the host explosive. We applied this tool to show the intermolecular interaction variation after insertion of the guest molecules in CL-20. The graph in Figure 5 changes as different guests are embedded; there is a noticeable decrease in symmetry about the x/y diagonal in the following order: CO 2  usually appear in energetic crystal [33][34][35], can be readily understood by the internal moieties of CL-20 being dominated by the O atom. This is one of the factors that determine the high energy release of CL-20. The O atoms with high negative ESPs (shown in Figure 1      The 2-D fingerprint plot directly demonstrates the intermolecular interactions of internal and external distances of atoms from the surface. It is possible to show the range of structures by the changes in the fingerprint plots while adding the guest into the host explosive. We applied this tool to show the intermolecular interaction variation after insertion of the guest molecules in CL-20. The graph in Figure 5 changes as different guests are embedded; there is a noticeable decrease in symmetry about the x/y diagonal in the following order: CO2, H2O, N2O, and lastly, H2O2, especially for the fingerprint of CL-20/H2O2. For pure CL-20, Figure 6 shows that intermolecular interactions are governed by O…O contacts. The O…O interactions that usually appear in energetic crystal [33][34][35], can be readily understood by the internal moieties of CL-20 being dominated by the O atom. This is one of the factors that determine the high energy release of CL-20. The O atoms with high negative ESPs (shown in Figure 1) to form O…O contacts can lead to a big electrostatic repulsion. This is not conducive to the stability of CL-20. The steric in the cage and rings is also shown in the plots of IRI and RDG isosurface. The other two main intermolecular interactions are H…O and O…H contacts. The existence of hydrogen bonds is beneficial to the stability of CL-20. The O…O contacts percentage contribution to the Hirshfeld surface decreases in the order of: CL-20 (44.2%), CL-20/CO2 (42.4%), CL-20/H2O2 (37.6%), CL-20/N2O (36.8%), and CL-20/H2O (35.9%). This implies that the repulsion in the cage and rings may decrease with the same order. The intuitive exhibition of repulsion in the cage and rings are shown in the plots of the NCI isosurface. The O…H and H…O contacts percentage contribution of CL-20/H2O2 is larger than for CL-20. By combining the decreasing O…O contacts and the increasing HBs, it can be inferred that CL-20/H2O2 is more stable than CL-20.  The inter-fragment interaction between all the defined fragments (one fragment is the host CL-20, the other fragment is one of the small guest molecules) can easily be recognized by the electrostatic interaction energy. The electrostatic interaction energies between guest molecules and the selected atoms of CL-20 are shown in Table 1 This contributes greatly to the sum of the electrostatic effect with binding (−0.14, −8.93, −0.11 kJ·mol −1 ). Meanwhile, the attraction between H 2 O 2 and CL-20 is very obvious and cannot be ignored, as the binding energy is much less than 0. This may be because of the different electrostatic potential (ESP) and van der Waals (vdW) potential of CL-20/H 2 O 2 contrary to the other complexes. However, the mutual repulsion of the fragment CO 2 and CL-20 contributes greatly to the sum of the electrostatic effect without binding (0.03 kJ·mol −1 ). Therefore, the CL-20 fragment combined with N 2 O, H 2 O 2 , H 2 O fragments may be a whole molecule. The CL-20 fragment with CO 2 may be two fragments. This conclusion confirms the two parts of the Hirshfeld surface for CL-20/CO 2 , as shown in The inter-fragment interaction between all the defined fragments (one fragment is the host CL-20, the other fragment is one of the small guest molecules) can easily be recognized by the electrostatic interaction energy. The electrostatic interaction energies between guest molecules and the selected atoms of CL-20 are shown in Table 1, with a calculation of which are closer to the guest molecules. The most important contribution to the attraction is the electrostatic interaction of the O…H, O…N and O…C contacts; this result is easy to understand since O is the acceptor atom of these contacts. However, even though there were rejections between O…O contacts and O…N contacts, the fragments between N2O, H2O2, H2O and CL-20 attract each other. This contributes greatly to the sum of the electrostatic effect with binding (−0.14, −8.93, −0.11 kJ·mol −1 ). Meanwhile, the attraction between H2O2 and CL-20 is very obvious and cannot be ignored, as the binding energy is much less than 0. This may be because of the different electrostatic potential (ESP) and van der Waals (vdW) potential of CL-20/H2O2 contrary to the other complexes. However, the mutual repulsion of the fragment CO2 and CL-20 contributes greatly to the sum of the electrostatic effect without binding (0.03 kJ·mol −1 ). Therefore, the CL-20 fragment combined with N2O, H2O2, H2O fragments may be a whole molecule. The CL-20 fragment with CO2 may be two fragments. This conclusion confirms the two parts of the Hirshfeld surface for CL-20/CO2, as shown in Figure 4. The two different kinds of interaction between CL-20/N2O, CL-20/H2O2, CL-20/H2O and CL-20/CO2 may be caused by the polarity of the guest molecules.

Electrostatic and vdW Interaction Characteristics of Host and Guest Molecules
The electrostatic potential (ESP) on the molecular vdW surface was used to study and predict intermolecular interaction, for example, the information of the close contact site, structure property and special hydrogen bonding [36][37][38], which is usually employed to study the molecular packing in cocrystals [39]. Therefore, it is very useful to investigate the important interaction between the host explosive and small guest molecules. The ESP-mapped vdW surface, in addition to the surface extrema of CL-20 and its host-guest complexes, are shown in Figure 7, and their surface areas are plotted as shown in Figure 8.

Electrostatic and vdW Interaction Characteristics of Host and Guest Molecules
The electrostatic potential (ESP) on the molecular vdW surface was used to study and predict intermolecular interaction, for example, the information of the close contact site, structure property and special hydrogen bonding [36][37][38], which is usually employed to study the molecular packing in cocrystals [39]. Therefore, it is very useful to investigate the important interaction between the host explosive and small guest molecules. The ESPmapped vdW surface, in addition to the surface extrema of CL-20 and its host-guest complexes, are shown in Figure 7, and their surface areas are plotted as shown in Figure 8.    Table 1. From the graph for CL-20 (Figure 9a), it is clear that the C and H atoms form bonds. The C-H fragments are overall positively charged because they largely intersect solid contour lines of the vdW surface close to the two C-H fragments. This shows that the C-H fragments are surrounded by positive value lines, and suggests that the C-H segments are more susceptible to electrophilic reactions that are more stable when they receive hydrion. While embedding different small molecules into the crystal lattice cavity of α-CL-20, the symmetry of the contour map of electrostatic potential is broken, and the contour lines become chaotic. This may be because the symmetry of CL-20 is broken. This shows that all host-guest complexes contribute to electrophilic reactions in the same manner as that of CL-20. However, the reaction sites may be a little changeable for the different intersect    The color-filled NCI isosurface not only demonstrates where weak interaction occurs, but is also an intuitive presentation of their interaction-such as repulsion or attraction-and their magnitude. We can identify different types of regions by simply examining their colors. Recalling the color scale bar shown previously in Figure 10A, more blue implies a stronger attractive interaction. The elliptical slab between the oxygen and hydrogen atoms shows green color in Figure 10, so we can conclude that there exists a hydrogen bond, but not a very strong one. The yellow circle demonstrates the vdW interaction region, which shows that the electron density in this region is low. Obviously, the regions at the center of the cage and rings correspond to strong steric interaction, since they are filled by red. This result explains the relatively low stability of CL-20. The configuration of the CL-20 fragment changes significantly in the host-guest complex, due to the appearance of the vdW interaction region between the two NO 2 fragments. The configuration caused by the repulsion of guest molecules and NO 2 prepares enough space to accommodate guest molecules. The red color becomes lighter and the shape becomes thinner at the center of the cage in the host-guest complex. This demonstrates that the cage of CL-20 fragments in the host-guest complex is more stable than the CL-20. This conclusion is consistent with the finding that the CL-20/N 2 O has higher thermal stability and lower impact sensitivity than CL-20 [39,40]. IRI is not only able to reveal the weak interaction region, but also shows the steric effect within the cage and rings of CL-20 [8]. There are four obvious spikes below the horizontal isosurface line of RDG = 0.9, and the spikes can be classified into three types for CL-20 and its complexes [41]. The spike at about −0.275 a.u. indicates the existence of hydrogen bonds. At about −0.02 a.u. and 0.02 a.u., there are two spikes, which demonstrate that the complexes have vdW interaction. The steric effect in the cage and ring is displayed when the spike appears at about 0.275 a.u. This phenomenon explains the instability of CL-20. The influence of small guest molecules on the intramolecular interaction of CL-20 is shown in Figure 11a. In addition, the strength of the weak interaction has a positive correlation with electron density in the corresponding region. The IRI plots of CL-  IRI is not only able to reveal the weak interaction region, but also shows the steric effect within the cage and rings of CL-20 [8]. There are four obvious spikes below the horizontal isosurface line of RDG = 0.9, and the spikes can be classified into three types for CL-20 and its complexes [41]. The spike at about −0.275 a.u. indicates the existence of hydrogen bonds. At about −0.02 a.u. and 0.02 a.u., there are two spikes, which demonstrate that the complexes have vdW interaction. The steric effect in the cage and ring is displayed when the spike appears at about 0.275 a.u. This phenomenon explains the instability of CL-20. The influence of small guest molecules on the intramolecular interaction of CL-20 is shown in Figure 11a Table 1 demonstrates that the summation of H…Guest electrostatic interaction energy of CL-20/H2O, CL-20/CO2 and CL-20/N2O is much less than one H…H2O2 (−3.77 kJ·mol −1 ) electrostatic interaction energy. It also shows that the addition of H2O, CO2, and N2O weakens the intermolecular hydrogen bonds. and CL-20/H2O2 (19.5 kcal·mol −1 ) correspond to the macropolar host-guest complex more than CL-20 (18.61 kcal·mol −1 ). CL-20/CO2 (14.95 kcal·mol −1 ) has the smallest polarity. The polarities of CL-20 and its host-guest complexes are rather high, since their MPIs are higher than benzene (8.4 kcal·mol −1 ) [42], which possesses the common unsaturated hydrocarbons as determined by experimental chemists. This further implies that the strength of the electrostatic interaction between the CL-20 and its host-guest complex should be fairly strong. The stronger the electrostatic interaction is, the more stable the explosive is. The results are consistent with the conclusion shown by the color-filled NCI isosurface.
Owing to the relatively low polarity of CL-20/CO2, the vdW interaction [43] of CL-20/CO2 is likely to be very important with regard to its electrostatic interaction in intermolecular complexation. In Figure 12, the green isosurface represents the region where vdW is negative. The guest molecules tend to be attracted to the green region due to the driven force of dispersion attraction. The region close to the nuclei is fully enclosed by The polarities of CL-20 and its host-guest complexes are rather high, since their MPIs are higher than benzene (8.4 kcal·mol −1 ) [42], which possesses the common unsaturated hydrocarbons as determined by experimental chemists. This further implies that the strength of the electrostatic interaction between the CL-20 and its host-guest complex should be fairly strong. The stronger the electrostatic interaction is, the more stable the explosive is. The results are consistent with the conclusion shown by the color-filled NCI isosurface.
Owing to the relatively low polarity of CL-20/CO 2 , the vdW interaction [43] of CL-20/CO 2 is likely to be very important with regard to its electrostatic interaction in intermolecular complexation. In Figure 12, the green isosurface represents the region where vdW is negative. The guest molecules tend to be attracted to the green region due to the driven force of dispersion attraction. The region close to the nuclei is fully enclosed by blue isosurface, indicating that the exchange repulsion potential dominates the vdW potential in this area. The surfaces of the vdW potential show that CL-20/H 2 O 2 is a whole, while other host-guest complexes are comprised of two parts. This contributes to the closer distance between CL-20 and H 2 O 2 . This is also intuitively shown in the color-filled NCI isosurface. It further explains that the CL-20/H 2 O 2 is the special existence for the other complex.

Effect on the Chemical Bonding of CL-20 by the Small Molecules
The electronic structure, shown in Figure 13, and the charge, change [44] before and after embedding different small molecules into the crystal lattice cavity of α-CL-20. The yellow isosurface (0.001 a.u.) represents the region in which the electron density of the bonds increases. As shown in Figure 13a, it is obvious that electron density shifts from C-H fragments toward N-NO2 fragments to strengthen the bonding energy. The electrons mainly accumulate in the branch chain of CL-20. The electronegativity of O atoms of the nitro branch chain is larger than that of N and C atoms, causing the electron cloud density of the entire cage structure to be biased towards O atoms, thus presenting greater electronegativity. The electronegativity of N atoms is smaller than that of O atoms, and the electronegativity of N atoms is larger than that of the cage structure. This demonstrates that the N-NO2 bonds are more active than the N-O bonds. This is consistent with the conclusion that CL-20 has only one distinct initial decomposition channel homolysis of the N-NO2 bond. The cages of other atoms are electropositive. The positive electricity compared with the smaller distance between atoms by the cage structure, further intensifies the mutually repulsive force of the cage structure. This is an important factor in determining the instability of CL-20. The result corresponds with the incidental phase transformations [45,46], and the cage collapse with the C-N bonds rupture [47]. Table 2 lists the charge density of each atom for CL-20 and its complexes. The variation tendency of charge density between CL-20/CO2 and CL-20/N2O are similar, while it is slightly different to CL-20. The variation tendency of charge density between CL-20/H2O and CL-20/H2O2 is similar. However, the variation trend is very different from CL-20. This indicates that CO2 and N2O have little effect on the charge density of CL-20, while H2O and H2O2 have a larger effect on the charge density of CL-20. This difference may depend on whether the guest molecule contains hydrogen atoms.

Effect on the Chemical Bonding of CL-20 by the Small Molecules
The electronic structure, shown in Figure 13, and the charge, change [44] before and after embedding different small molecules into the crystal lattice cavity of α-CL-20. The yellow isosurface (0.001 a.u.) represents the region in which the electron density of the bonds increases. As shown in Figure 13a, it is obvious that electron density shifts from C-H fragments toward N-NO 2 fragments to strengthen the bonding energy. The electrons mainly accumulate in the branch chain of CL-20. The electronegativity of O atoms of the nitro branch chain is larger than that of N and C atoms, causing the electron cloud density of the entire cage structure to be biased towards O atoms, thus presenting greater electronegativity. The electronegativity of N atoms is smaller than that of O atoms, and the electronegativity of N atoms is larger than that of the cage structure. This demonstrates that the N-NO 2 bonds are more active than the N-O bonds. This is consistent with the conclusion that CL-20 has only one distinct initial decomposition channel homolysis of the N-NO 2 bond. The cages of other atoms are electropositive. The positive electricity compared with the smaller distance between atoms by the cage structure, further intensifies the mutually repulsive force of the cage structure. This is an important factor in determining the instability of CL-20. The result corresponds with the incidental phase transformations [45,46], and the cage collapse with the C-N bonds rupture [47]. Table 2     The differential charge densities of CL-20 and its complexes are shown in Figure 14. For the H 2 O 2 embedded in CL-20, the charge density variation of CL-20 is more obvious than for the other complexes. The charge density variation of the three NO 2 fragments close to H 2 O 2 is remarkable. The one NO 2 fragment away from the inserted H 2 O 2 obtains more electrons than the two NO 2 fragments close to the H 2 O 2 . For the two NO 2 fragments, the two O atoms adjacent to the H atoms of H 2 O 2 lose electrons. Therefore, the electrostatic interaction between the host and guest molecules is enhanced, which results in a decrease in the distance, and an increased intermolecular interaction. This is consistent with the charge density presented in Table 2 Only the O atoms of CO 2 obtain electrons from the C atoms of CO 2 . The electrostatic interaction between CO 2 and CL-20 is unchanged, which results in a large distance, as also shown in Figures 4 and 10. As a result, the charge density of CL-20 changes obviously with the H 2 O and H 2 O 2 insertions, although the charge density of CL-20 changes little for the N 2 O and CO 2 insertions. The results also indicate that the distribution of the electrons transferred to the CL-20 is destroyed in different degrees due to the different guests. This demonstrates that the charge density variation is decided by the hydrogen contained by, and the oxidability of, guest molecules.
The COHP technique can be qualitative and correctly describes negative (i.e., bonding) and positive (i.e., antibonding) contributions [48,49] of chemical bonding with bandstructure energy, as shown in Figure 15. The larger the value of the area above 0 minus the area below 0, the more bonds there are. The value of -COHP decreases according to the N-O of NO 2 , N-NO 2 , C-N and C-C, as shown in Table 3. This demonstrates the cage is relatively more unstable than other chains of CL-20. Where the distance between two atoms exceeds 1.6 Å, the value of their -COHP is positive, which shows that the intermolecular interactions are not negligible for CL-20. This demonstrates the relatively unstable of CL-20.
In order to visually and simply show the effect of small guest molecules on the COHP of CL-20, we divided the bonds into two types. The first type is cage bonds such as C-C bonds and C-N bonds. The second type is branched chain bonds such as C-H bonds, N-N bonds and N-O bonds. For CL-20/2H 2 O 2 , the -COHP of C-C bonds is significantly higher. It is a little higher for CL-20/H 2 O. However, it is lower relative to CL-20/CO 2 and CL-20/N 2 O. The -COHP of C-N bonds is substantially higher for CL-20/2H 2

Conclusions
We conducted a systematic and in-depth theoretical exploration and comparison of the intermolecular contacts, intermolecular interaction characteristics and chemical bonding analysis of the high-energy explosive CL-20 and the new host-guest explosives CL-20/H 2 O 2 , CL-20/H 2 O, CL-20/CO 2 and CL-20/N 2 O. The main findings and conclusions are summarized as follows: (1) The d norm Hirshfeld surfaces, 2-D fingerprint plot and individual atomic contact percentage contribution demonstrate that the cage of CL-20 fragments in host-guest complexes are more stable than CL-20. The electrostatic interaction energy shows that CL-20/H 2 O 2 possesses stronger hydrogen bonds and stronger mutual attraction between host and guest molecules than other complexes. The specific distribution of host and guest molecules are affected by the polarity and oxidizability of the guest molecules; The results of this study revealed that the guest H 2 O 2 small molecule played a certain stable role for CL-20. For the synthesis of new energetic materials with host-guest inclusion strategy, we investigated new guest molecules by referencing properties such as geometry configuration, oxidability, polarity and hydrogen content of H 2 O 2 . Our results provide fundamental insight into the roles of guest molecules in host-guest crystals and may be helpful for the formation of new host-guest energetic materials by incorporating appropriate species of small molecules into crystal lattice voids.