Investigation on Adsorption and Decomposition Properties of CL-20/FOX-7 Molecules on MgH2(110) Surface by First-Principles

Metal hydrides are regarded as promising hydrogen-supplying fuel for energetic materials while CL-20 (Hexanitrohexaazaisowurtzitane) and FOX-7 (1,1-Diamino-2,2-dinitroethylene) are typical principal components commonly used in energetic materials. Hence, it is interesting to explore the interactions between them for development of new energetic systems. In this paper, the adsorption and decomposition of CL-20 or FOX-7 molecules on the MgH2 (110) crystal surface were investigated by employing the First-Principles. In total, 18 adsorption configurations for CL-20/MgH2 (110) and 12 adsorption configurations for FOX-7/MgH2 (110) were considered. The geometric parameters for the configurations, adsorption energies, charge transfer, density of states, and decomposition mechanism were obtained and analyzed. In most of the configurations, chemical adsorption will occur. Moreover, the orientation of the nitro-group in CL-20 or FOX-7 with regard to the MgH2 (110) surface plays an important role on whether and how the energetic molecule decomposes. The adsorption and decomposition of CL-20 or FOX-7 on MgH2 could be attributed to the strong charge transfer between Mg atoms in the first layer of MgH2 (110) surface and oxygen as well as nitrogen atoms in the nitro-group of CL-20 or FOX-7 molecules.


Introduction
With the development of hydrogen energy, various hydrogen storage materials emerge such as metal hydrides. One notable application of metal hydrides is the additive in energetic materials such as solid propellants and explosives [1][2][3][4][5][6]. Metal hydrides, which are commonly considered as energy carriers for the hydrogen economy [7], have also attracted attention in solid propulsion due to their high chemical energy and remarkable activity [8]. The hydrogen offered by metal hydrides via dehydrogenation reactions is found effective in reducing the relative molecular mass of gas products of combustion, which favours the increase of specific impulses [9,10] for propellants. Furthermore, oxidation of the metal and H 2 , which are the products of the dehydrogenation reaction, could also provide a large amount of energy. The metal hydrides could be added into propellants as high energy combustion agents, and the energy level of propellant can be improved. In a previous experimental study by the authors [11], the combustion characteristics were found improved when adding ZrH 2 in a double-base propellant. Similarly, the addition of hydrogen storage materials in explosives can increase All calculations were performed by employing the CASTEP (Cambridge Sequential Total 94 Energy Package) program [37] with Vanderbilt-type ultrasoft pseudo potentials [38] and a 95 plane-wave expansion of the wave functions in the software package Materials Studio 8.0. 96 General gradient approximation (GGA) was adopted in the exchange and correlation 97 interactions. PBE (the functional form proposed by Perdew, Burke, and Ernzerhof [39]) was 98 employed. The electronic wave functions were obtained by a density-mixing scheme [40] and 99 the structures were relaxed using the Broyden, Fletcher, Goldfarb, and Shannon method [41]. 100 The cut-off energy was set as 380 eV and the k-point sampling was set as 2 × 2 × 1, which 101 showed good convergence for energy, geometry, and force. When the interatomic interaction 102 force is less than 0.05 eV/Å, the stress is less than 0.1 GPa, the change in atomic energy is less 103 than 2.0 × 10 −5 eV/atom, and the change in displacement is less than 0.002 Å. The condition of 104 convergence was deemed to be met. Spin polarization was not considered in the calculation. 105

Computational Model 106
MgH2 has three crystal morphologies [42,43], and the α-form among them is the most 107 stable at normal temperature, whose (110) face is most stable [44]. Therefore, the computational 108 model employed a 4 × 2 × 1 supercell and a nine-layer MgH2 (110) surface (as shown in Figure  109 1). The cell size with a rhombic box of a × b × c was 12.90 Å × 12.04 Å × 30. 15 Å. 110 Five polymorphs, α to ε, are known for CL-20. The ε-polymorph is stable at room 111 temperature and shows the highest density [45]. One ε-CL-20 molecule [46,47] (as in Figure 2) 112 was placed on the upper side of the MgH2 (110) surface. Two types of nitro groups exist in the 113 CL-20 molecule. The one attached to the six-member ring of CL-20 was represented by type-A, 114 and the other attached to the five-member ring of CL-20, which was represented by type-B (see 115 Figure 2). 116 1,1-Diamino-2,2-dinitroethylene (FOX-7) is a novel high energetic ingredient [48], whose 117 structure [49,50] was shown in

Adsorption
Configuration It can be seen that the closer the nitro group of CL-20 is to the MgH 2 (110) surface, the easier it will be to get the corresponding bonds elongated or ruptured. Besides, there are no bonds rupture or formation in the b, c, d, f, and k configurations, and physical adsorption are deemed to occur. For the rest of the configurations with decomposition, the bond rupture occurs mostly in the mono-N-NO 2 .
The geometrical parameters of the 12 FOX-7/MgH 2 (110) configurations are shown in Table 2, where similar notations for the bond length in the CL-20/MgH 2 system applies to r (N1-O1) , r (N1-O2) , and r (C1-N1) in the FOX-7/MgH 2 system. Before the adsorption, r (N1-O1)0 =1.242 Å, r (N1-O2)0 =1.250 Å, and r (C1-N1)0 =1.416 Å. Rupture of the bonds suggesting chemical adsorption occurs in the F-V1, F-V2, F-V5, F-V6, F-P2, and F-P5 configurations. Instead, the physical adsorption occurs in the rest of the six configurations. The bond rupture mainly occurs in the mono-nitro-N-O bond. This is followed by the bis-nitro-N-O bond, and the C-N bond does not rupture at all, which illustrates an increasing stability of the corresponding bonds. In general, the bonds of the FOX-7 molecule show more tendency to rupture the FV-type configurations, which exhibits an impact of nitro-bond orientation on the interaction between FOX-7 and MgH 2 .

Adsorption Energies
The adsorption energies (E ad ) of CL20 and FOX-7 molecules on the MgH 2 (110) surface were calculated. E ad is defined as: where the E slab/molecule is the total energy of the adsorption configurations after adsorption. E slab is the single point energy of the MgH 2 (110) surface and E slab = −48267.4eV. The E molecule is the single point energy of the energetic material molecule, For the CL-20/MgH 2 (110) system, adsorption energies E ad for the 18 adsorption configurations are shown in Figure 5.

Adsorption Energies 172
The adsorption energies (Ead) of CL20 and FOX-7 molecules on the MgH2 (110) surface 173 were calculated. Ead is defined as: 174 Where the Eslab/molecule is the total energy of the adsorption configurations after adsorption.

182
In the case of (f) configuration (type-A nitro vertical to hole), whose adsorption energy is 183 the lowest (−10.5 eV), no bond rupture or formation occurs and, hence, physical adsorption is 184 expected. However, in the case of (n) configuration (type-B nitro parallel to H top), whose 185 adsorption energy is the highest (−21.8 eV), the mono-N-NO2 bond and mono-nitro mono-N-O 186 bond of the type B nitro group rupture, producing NO2, oxygen atom, and CL-20 fragment. 187 The greater the adsorption energy is, the more intense the corresponding interaction will be. 188 Moreover, we can see that, at the same adsorption sites, the adsorption energies of 189 configurations with type-B nitro adsorbed are generally larger than the configurations with In the case of (f) configuration (type-A nitro vertical to hole), whose adsorption energy is the lowest (−10.5 eV), no bond rupture or formation occurs and, hence, physical adsorption is expected. However, in the case of (n) configuration (type-B nitro parallel to H top), whose adsorption energy is the highest (−21.8 eV), the mono-N-NO 2 bond and mono-nitro mono-N-O bond of the type B nitro group rupture, producing NO 2 , oxygen atom, and CL-20 fragment. The greater the adsorption energy is, the more intense the corresponding interaction will be. Moreover, we can see that, at the same adsorption sites, the adsorption energies of configurations with type-B nitro adsorbed are generally larger than the configurations with type-A nitro adsorbed. It means that the type-B nitro are easier to adsorb on the MgH 2 (110) surface than type-A nitro. For type-B nitro of CL-20, at the same adsorption sites, the adsorption energies of adsorption configurations where adsorbed nitro of CL-20 is parallel to MgH 2 (110) are larger than the adsorption configurations where adsorbed nitro is perpendicular to MgH 2 (110). It means that CL-20 molecule is easier to adsorb on the surface of MgH 2 (110) when its nitro group is placed horizontally than being placed vertically.
Meanwhile, the adsorption energies E ad of FOX-7/MgH 2 (110) adsorption configurations are shown in Figure 6. The negative adsorption energies for all the configurations, similar to the CL-20/MgH 2 system, indicate exothermic and stable adsorption [51]. At the six adsorption sites, the adsorption energies of the FV-type configurations are unanimously greater than those of the FP-type configurations, which corresponds to a more stable adsorption. The highest adsorption energy is −21.2 eV when the nitro is vertical to the Mg-H bridge (F-V5 configuration), and the lowest adsorption energy is -15.9 eV when the nitro is parallel to the H-H bridge (F-P3 configuration).
Molecules 2020, 25, x FOR PEER REVIEW` 7 of 17 are larger than the adsorption configurations where adsorbed nitro is perpendicular to MgH2 194 (110). It means that CL-20 molecule is easier to adsorb on the surface of MgH2 (110) when its 195 nitro group is placed horizontally than being placed vertically. 196 Meanwhile, the adsorption energies Ead of FOX-7/MgH2 (110) adsorption configurations 197 are shown in Figure 6. The negative adsorption energies for all the configurations, similar to 198 the CL-20/MgH2 system, indicate exothermic and stable adsorption [51]. At the six adsorption 199 sites, the adsorption energies of the FV-type configurations are unanimously greater than those 200 of the FP-type configurations, which corresponds to a more stable adsorption.

Charge Transfer of Adsorption Configurations
Electron delocalization and charge transfer induce a chemical reaction of a system. In this section the charge transfer between Mg, H atoms in the first layer of the MgH 2 (110) crystal face, and the activation centers of O and N atoms in the CL-20 or FOX-7 molecules is analyzed. Table 3

239
The charge distribution of Mg and H atoms in the first layer, and the activation center O1, 240 O2, and N1 atoms in the FOX-7 molecule of the FOX-7/MgH2 (110) system before and after 241 adsorption are shown in Table 4. It can be seen that the charge of Mg atoms is increased after 242 adsorption, while the charges of O1, O2, and N1 atoms are decreased. However, the charges of 243 The charge distribution of Mg and H atoms in the first layer, and the activation center O1, O2, and N1 atoms in the FOX-7 molecule of the FOX-7/MgH 2 (110) system before and after adsorption are shown in Table 4. It can be seen that the charge of Mg atoms is increased after adsorption, while the charges of O1, O2, and N1 atoms are decreased. However, the charges of H atoms and C1 atoms show little changes. Apparently, strong charge transfer mainly occurs between Mg atoms and O, N atoms of the adsorbed nitro group in the FOX-7 molecule.

Density of States of Adsorption Configurations
In order to further investigate the interaction mechanism of the CL-20 or FOX-7 molecule with MgH 2 , the density of states (DOS) and partial density of states (PDOS) of the involving systems were analyzed. The DOS of MgH 2 , CL-20, and 18 configurations of CL-20/MgH 2 (110), were shown in Figure 8. First, MgH 2 shows DOS around two energy levels, which include −44.7eV~−40.3 eV and −9.2 eV~4.2 eV. The results are close to those reported in Reference [52], verifying the slab model and parameters used. Second, the DOS for the 18 MgH 2 /CL-20 configurations are mainly distributed near three energy levels, −44.5~−41.0 eV, −12.1~−0.5 eV, and 0~5.5 eV. In proximity to the Fermi level, the region with the strongest DOS for CL-20 nearly completely coincides with the local DOS for MgH 2 , which indicates that the orbits of both are prone to mixing and hybridization. Therefore, the density of states near the Fermi level of all the 18 configurations has intensity significantly higher than those of CL-20 molecules or the MgH 2 (110) crystal face. Total density of states of Mg and H atoms in MgH 2 (110) slab and the O and N atoms of CL-20 before adsorption are shown in Figure 9. The DOS after adsorption in configurations (g) are shown in Figure 10. We can see that the three peaks of Mg atoms near −44.5~−40.5eV level are merged into one peak because the adsorbed CL-20 molecule causes the breakage of the original periodicity of the crystal structure of magnesium hydride. In addition, the DOS of O and N atoms move toward the lower energy level. Further analysis proves that the density of states of the Fermi level is mainly contributed by p orbits of Mg atoms, p orbits of N atoms, and O atoms of CL-20 molecules. The mixing and hybridization effect of the p orbits enhances the electron delocalization, promotes charge transfer, and, ultimately, leads to decomposition of CL-20 molecules on the MgH 2 (110) crystal face. In order to investigate why the decomposition of FOX-7 on the MgH 2 (110) surface is mainly caused by N-O bond rupture, its partial density of states (PDOS) was further investigated. The PDOS of Mg, H, O1, O2, and C1 atoms in F-V1 configurations are shown in Figure 11.
According to PDOS, it can be seen that the p orbital energy of Mg atoms and C1 atoms are located on both sides of the Fermi level, respectively, while the p orbital energy of O1, O2, and N1 atoms crosses the Fermi level. Apparently, Mg, O1, O2, and N1 atoms all have peaks in the vicinity of the Fermi energy levels. In addition, The DOS of Mg, O1, O2, and N1 orbital hybridization is likely to occur, which leads to strong interactions between Mg, O1, O2, and N1 atoms and promotes FOX-7 molecule adsorption on the surface of magnesium hydride. This also confirms the strong charge transfer between Mg and O, N mentioned previously. partial density of states (PDOS).

279
In order to investigate why the decomposition of FOX-7 on the MgH2(110) surface is 280 mainly caused by N-O bond rupture, its partial density of states (PDOS) was further 281 investigated. The PDOS of Mg, H, O1, O2, and C1 atoms in F-V1 configurations are shown in 282 Figure 11.

Decomposition Mechanisms
Based on the previously mentioned calculations, the decomposition pathways of CL-20 or FOX-7 molecules on the MgH 2 (110) surface were obtained.

(I) CL-20 decomposition mechanism
In total, five different decomposition mechanisms have been found for CL-20 (see Figure 12). Since there is more charge transfer between N atoms and Mg atoms than between O atoms and Mg atoms, N-NO 2 is more likely to be ruptured than N-O of nitro when the CL-20 molecule is absorbed on the surface of MgH 2 (110).
Molecules 2020, 25, x FOR PEER REVIEW` 12 of 17 In total, five different decomposition mechanisms have been found for CL-20 (see Figure  297 12). Since there is more charge transfer between N atoms and Mg atoms than between O atoms 298 and Mg atoms, N-NO2 is more likely to be ruptured than N-O of nitro when the CL-20 molecule 299 is absorbed on the surface of MgH2(110). 300

303
(1) The mono-N-NO2 bond of type A nitro rupture involves the N-NO2 bond (attaching to 304 type A nitro) in the symmetric position of N1-N2 bond being ruptured, which produces an NO2 305 and CL-20-1 fragment. This is applicable to the adsorption configurations of (a), and (e). 306 (2) The bis-N-NO2 bond of type B nitro rupture involves the bis-N-NO2 bond of type B 307 nitro being ruptured, which produces two NO2 fragments and one CL-20-2 fragment. This is 308 applicable to the adsorption configurations of (m), (o), and (p). 309 ( The mono-N-NO2 bond of type B nitro is ruptured, which produces an NO2 and CL-20-3 312 fragment. This is applicable to the adsorption configurations of (n). (1) The mono-N-NO 2 bond of type A nitro rupture involves the N-NO 2 bond (attaching to type A nitro) in the symmetric position of N1-N2 bond being ruptured, which produces an NO 2 and CL-20-1 fragment. This is applicable to the adsorption configurations of (a), and (e).
(2) The bis-N-NO 2 bond of type B nitro rupture involves the bis-N-NO 2 bond of type B nitro being ruptured, which produces two NO 2 fragments and one CL-20-2 fragment. This is applicable to the adsorption configurations of (m), (o), and (p).
(3) The mono-N-NO 2 bond of type B nitro and mono-nitro mono-N-O bond of type B nitro rupture involves mono-nitro mono-N-O bond of type B nitro being ruptured and producing O.
The mono-N-NO 2 bond of type B nitro is ruptured, which produces an NO 2 and CL-20-3 fragment. This is applicable to the adsorption configurations of (n).
(4) The mono-N-NO 2 bond of type B nitro rupture includes the mono-N-NO 2 bond of type B nitro being ruptured, which produces the NO 2 and CL-20-4 fragment. This is applicable to the adsorption configurations of (h), (j), (l), (q), and (r).
(5) The mono-nitro mono-N-O bond of type B nitro rupture includes the mono-nitro mono-N-O bond of type B nitro being ruptured, which produces the O and CL-20-5 fragment. This is applicable to the adsorption configurations of (g) and (i).
(II) FOX-7 decomposition mechanism For the decomposition of FOX-7 on the MgH 2 (110) surface, three different pathways were found. This is shown in Figure 13. to the morphology reported in Reference [29]. The analysis of gas-phase products was carried 337 out using a fast scanning Fourier transform infrared spectrometer (Nicolet 5700FTIR). The 338 interference shown in Figure 15. At a low temperature under 510.1K, the mixture produces NO2 with steady 345 low content (less than 5 × 10 −13 ). Beyond 510.1K, the content of NO2 increases sharply. The peak 346 value of CL-20/MgH2 mixture is 2.1 × 10 −12 at 540.7 K, and FOX-7/MgH2 mixture is 1.9 × 10 −12 at 347 594.7 K. CL-20/MgH2 mixture can produce more NO2 at a lower temperature than the FOX-348 7/MgH2 mixture, which shows higher reactivity for thermal decomposition. (1) The mono-nitro mono-N-O bond rupture involves the FOX-7-1 fragment and one oxygen atom being formed, which is applicable to F-V1, F-V2, F-V5, and F-P2 configurations.
(2) The mono-nitro bis-N-O bonds, mono-nitro mono-N-O bonds, and mono N-H bond rupture. The FOX-7-2 fragment, one oxygen atom, and one OH are formed, which is applicable to F-V6 configurations.
(3) Bis-nitro mono-N-O bonds rupture. The FOX-7-3 fragment and two oxygen atoms are formed, which is applicable to F-P5 configurations.
In addition, the T-Jump/FTIR combined technology was used to study the thermal decomposition of CL-20/MgH 2 and FOX-7/MgH 2 mixtures with a mass ratio of 1. The microscopic images of MgH 2 samples were obtained ( Figure 14) using the field emission scanning electron microscope Carl Zeiss SIGMA. MgH 2 shows a regular spherical shape close to the morphology reported in Reference [29]. The analysis of gas-phase products was carried out using a fast scanning Fourier transform infrared spectrometer (Nicolet 5700FTIR). The interference pattern of incident light was obtained by the Michelson interferometer. The spectral data range was 650 to 4 000 cm −1 . The time interval of rapid scanning thermal decomposition process data was 0.125 s. A high-purity argon atmosphere and normal pressure were applied in the experiments. The rapid thermal cracking process of the mixture of CL-20/MgH 2 and FOX-7/MgH 2 were studied under different temperatures and a heating rate of 10 K/min. The NO 2 content in the product of CL-20/MgH 2 and FOX-7/MgH 2 with temperature is shown in Figure 15. At a low temperature under 510.1K, the mixture produces NO 2 with steady low content (less than 5 × 10 −13 ). Beyond 510.1K, the content of NO 2 increases sharply. The peak value of CL-20/MgH 2 mixture is 2.1 × 10 −12 at 540.7 K, and FOX-7/MgH 2 mixture is 1.9 × 10 −12 at 594.7 K. CL-20/MgH 2 mixture can produce more NO 2 at a lower temperature than the FOX-7/MgH 2 mixture, which shows higher reactivity for thermal decomposition. 20/MgH2 and FOX-7/MgH2 were studied under different temperatures and a heating rate of 10 343 K/min. The NO2 content in the product of CL-20/MgH2 and FOX-7/MgH2 with temperature is 344 shown in Figure 15. At a low temperature under 510.1K, the mixture produces NO2 with steady 345 low content (less than 5 × 10 −13 ). Beyond 510.1K, the content of NO2 increases sharply. The peak 346 value of CL-20/MgH2 mixture is 2.1 × 10 −12 at 540.7 K, and FOX-7/MgH2 mixture is 1.9 × 10 −12 at 347 594.7 K. CL-20/MgH2 mixture can produce more NO2 at a lower temperature than the FOX-348 7/MgH2 mixture, which shows higher reactivity for thermal decomposition.

354
Combining the simulation and experimental results, better stability of the FOX-7/MgH2 355 than CL-20/MgH2 can be confirmed, which is consistent with the stability comparison between 356 FOX-7 and CL-20 [53,54], and may be attributed to the special π-packing structure and 357 hydrogen bonds of FOX-7 [49]. Furthermore, as pointed out in several previous studies [55-358 57], CL-20 decomposition generally starts from the fracture of the weakest bonds, i.e., mono-359 N-NO2, particularly those connecting the five-member ring and the type-B nitro group [58]. On 360 the other hand, the C-N bond connecting the nitro group in the FOX-7 is relatively stable and 361 not ready for rupture under normal conditions [59]. Such observations could account for the 362 varying tendency of decomposition for adsorptions of type A and B nitro in CL-20 on the MgH2 363 (110) surface, and the different products from decomposition of CL-20/MgH2 and FOX-7/MgH2. 364

Conclusions 365
The adsorption and decomposition of CL-20 or FOX-7 molecules on the MgH2(110) surface 366 were studied in this paper using the First Principles method. The above research showed that: 367 (1) The bonds of the adsorbed nitro group in energetic molecules are either ruptured or 368 elongated after adsorption, which corresponds to chemical or physical adsorptions, 369 respectively. Negative adsorption energies for all the concerning configurations indicate 370 exothermic and stable adsorption of CL-20 and FOX-7 molecules. The nitro groups attached to 371 the five-member ring of CL-20 (type B nitro) are easier to adsorb on the MgH2(110) surface than 372 the nitro group attached to the six-member ring of CL-20 (type A nitro). For the type-B nitro, 373 the adsorption is easier to proceed when the corresponding nitro bond is parallel rather than 374 perpendicular to the MgH2 (110) surface. On the other hand, chemical adsorption with bond 375 rupture is less likely to take place for FOX-7 than for CL-20, and the configurations with 376 adsorbed nitro perpendicular to the MgH2 (110) surface show more tendency for 377 decomposition. 378 (2) The adsorption and decomposition of energetic molecules (CL-20 or FOX-7) on the 379 Combining the simulation and experimental results, better stability of the FOX-7/MgH 2 than CL-20/MgH 2 can be confirmed, which is consistent with the stability comparison between FOX-7 and CL-20 [53,54], and may be attributed to the special π-packing structure and hydrogen bonds of FOX-7 [49]. Furthermore, as pointed out in several previous studies [55][56][57], CL-20 decomposition generally starts from the fracture of the weakest bonds, i.e., mono-N-NO 2 , particularly those connecting the five-member ring and the type-B nitro group [58]. On the other hand, the C-N bond connecting the nitro group in the FOX-7 is relatively stable and not ready for rupture under normal conditions [59]. Such observations could account for the varying tendency of decomposition for adsorptions of type A and B nitro in CL-20 on the MgH 2 (110) surface, and the different products from decomposition of CL-20/MgH 2 and FOX-7/MgH 2 .

Conclusions
The adsorption and decomposition of CL-20 or FOX-7 molecules on the MgH 2 (110) surface were studied in this paper using the First Principles method. The above research showed that: (1) The bonds of the adsorbed nitro group in energetic molecules are either ruptured or elongated after adsorption, which corresponds to chemical or physical adsorptions, respectively. Negative adsorption energies for all the concerning configurations indicate exothermic and stable adsorption of CL-20 and FOX-7 molecules. The nitro groups attached to the five-member ring of CL-20 (type B nitro) are easier to adsorb on the MgH 2 (110) surface than the nitro group attached to the six-member ring of CL-20 (type A nitro). For the type-B nitro, the adsorption is easier to proceed when the corresponding nitro bond is parallel rather than perpendicular to the MgH 2 (110) surface. On the other hand, chemical adsorption with bond rupture is less likely to take place for FOX-7 than for CL-20, and the configurations with adsorbed nitro perpendicular to the MgH 2 (110) surface show more tendency for decomposition.
(2) The adsorption and decomposition of energetic molecules (CL-20 or FOX-7) on the surface of MgH 2 (110) is closely related to the strong charge transfer between Mg atoms in the MgH 2 (110) surface and oxygen as well as the nitrogen atoms in the adsorbed nitro group of energetic molecules. Meanwhile, through the DOS of Mg, O, and N, we have found that orbital hybridization is likely to occur near the Fermi energy level, which promotes adsorption of energetic molecules on the surface of MgH 2 (110) and the fracture of bonds thereafter.
(3) In total, five decomposition mechanisms of CL-20 on the surface of MgH 2 (110) were determined for the 18 adsorption configurations under discussion in which the rupture of the mono N-NO 2 bond is mostly involved and, hence, the main products contain NO 2 , oxygen atoms, and energetic molecule fragments. While for FOX-7/MgH 2 (110) adsorption, three decomposition mechanisms of FOX-7 were found for the 12 adsorption configurations with the main products being oxygen atoms, OH, and FOX-7 fragments.