Effects of Different Guests on Pyrolysis Mechanism of α-CL−20/Guest at High Temperatures by Reactive Molecular Dynamics Simulations at High Temperatures

The host–guest inclusion strategy has the potential to surpass the limitations of energy density and suboptimal performances of single explosives. The guest molecules can not only enhance the detonation performance of host explosives but also can enhance their stability. Therefore, a deep analysis of the role of guest influence on the pyrolysis decomposition of the host–guest explosive is necessary. The whole decomposition reaction stage of CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH was calculated by ReaxFF-MD. The incorporation of CO2, N2O and NH2OH significantly increase the energy levels of CL-20. However, different guests have little influence on the initial decomposition paths of CL-20. The Ea1 and Ea2 values of CL-20/CO2, CL-20/N2O, CL-20/NH2OH systems are higher than the CL-20/H2O system. Clearly, incorporation of CO2, N2O, NH2OH can inhibit the initial decomposition and intermediate decomposition stage of CL-20/H2O. Guest molecules become heavily involved in the reaction and influence on the reaction rates. k1 of CL-20/N2O and CL-20/NH2OH systems are significantly larger than that of CL-20/H2O at high temperatures. k1 of CL-20/CO2 system is very complex, which can be affected deeply by temperatures. k2 of the CL-20/CO2, CL-20/N2O systems is significantly smaller than that of CL-20/H2O at high temperatures. k2 of CL-20/NH2OH system shows little difference at high temperatures. For the CL-20/CO2 system, the k3 value of CO2 is slightly higher than that for CL-20/H2O, CL-20/N2O, CL-20/NH2OH systems, while the k3 values of N2 and H2O are slightly smaller than that for the CL-20/H2O, CL-20/N2O, CL-20/NH2OH systems. For the CL-20/N2O system, the k3 value of CO2 is slightly smaller than that for CL-20/H2O, CL-20/CO2, CL-20/NH2OH systems. For the CL-20/NH2OH system, the k3 value of H2O is slightly larger than that for CL-20/H2O, CL-20/CO2, CL-20/N2O systems. These mechanisms revealed that CO2, N2O and NH2OH molecules inhibit the early stages of the initial decomposition of CL-20 and play an important role for the decomposition subsequently.


Introduction
Successful balance between high energy and safety of energetic materials is challenging due to the time-consuming and difficult nature of synthesizing new energetic materials. Host-guest energetic materials, as shown in Figure 1, by embedding hydrogen-[1 -3] or nitrogen-containing [4,5] oxidizing small molecules into the crystal lattice voids may achieve the highest possible energy density and the maximum possible chemical stability [6]. 2,4,6,8,10,4,6,8,10,, as one of the energetic materials with greatest detonation performances and highest density from synthesized compounds, has been widely studied recently [7]. CL-20 has five polymorphs (namely β, γ, ε, ζ and δ) and an α-form hydrate [8], in which the ε-CL-20 has the highest density (2.044 g·cm −3 ) [8], β-CL-20 and γ-CL-20 are the dominant configurations of CL-20 molecules found in co-crystals [9]. Thus, many approaches have been used to tune its  20), as one of the energetic materials with greatest detonation performances and highest density from synthesized compounds, has been widely studied recently [7]. CL-20 has five polymorphs (namely β, γ, ε, ζ and δ) and an α-form hydrate [8], in which the ε-CL-20 has the highest density (2.044 g·cm −3 ) [8], β-CL-20 and γ-CL-20 are the dominant configurations of CL-20 molecules found in co-crystals [9]. Thus, many approaches have been used to tune its performance, such as co-crystals [10][11][12][13], nanosized particles [14] and so on. The nanoscaling cocrystals demonstrate enhanced stability and high solubility of nano-particles [15]. However, the high production costs and easily phase transition limit the widespread use of CL-20 [16,17]. Fortunately, removal of H2O from hydrated α-CL-20 can retain the stacking model [18]. Then, small molecules are filled in the cavity structure region that removes H2O molecules to form α-CL-20 host-guest explosives [3][4][5], which also can be regarded as the solvates of CL-20 [19][20][21]. Bennion J. C. [3] synthesized two polymorphic hydrogen peroxide (HP) solvates of α-CL-20. Two α-CL-20/H2O2 host-guest compounds are scarcely changing the lattice volume of the α-CL-20/H2O and improving the energy. Thereafter, researches are focused on adopting a host-guest inclusion strategy to embed a suitable guest within the void by removing the water of α-CL-20/H2O. A series of α-CL-20-guest The simulation of α-CL-20/guest mainly focused on the mechanism of host-guest molecular interaction and the pyrolysis mechanism at high temperatures. The intermolecular interaction is the central scientific issue of energetic cocrystals. Guo [6] had reviewed some typical energetic inclusion compounds and their structures, intermolecular interactions, stabilities, and energy properties. It provides a method to predict appropriate size of guest incorporate into the cavities of the α-CL-20 crystal. The systematic studies [22][23][24] on the comparison of interaction between the host-guest energetic complexes are devoted to summarizing the influence of guest on the performance of α-CL-20. The results would prove fundamental to summarize the properties of guest for α-CL-20/guest with structural stability. Meanwhile, in order to deeply analyze the role of hydrogen-guest small molecules in the host-guest system, the initial decomposition reactions of ICM-102/HNO 3 [25], ICM-102/H 2 O 2 [26] with pure ICM-102 and CL-20/H 2 O 2 [27] at several high temperatures were systematically studied by molecular dynamics simulations. It was found that the addition of small guest molecules significantly increased the energy levels of ICM-102 and CL-20 but had little effect on the thermal stability of the host-guest system. The initial reaction path of ICM-102 molecule was not changed by HNO 3 and H 2 O 2 , but HNO 3 and H 2 O 2 promoted the decomposition of ICM-102 molecule in the subsequent decomposition process. With the increase of temperature, the influence of H 2 O 2 on the pyrolysis reaction of CL-20 weakens [28].
All the simulation researches provide information to understand the influence of guest for the host explosives. However, the role of different nitrogen-guest small molecules in the host-guest system has not been studied systematically at different high temperatures. At the same high temperature, when do the different guest molecules participate in the decomposition reaction of host-guest explosive and how do they affect the decomposition process mechanism of host-guest explosive? What is the influence of the same guest on the pyrolysis of the host-guest explosive at different high temperatures? Therefore, detailed studies of the mechanism of the α-CL-20/nitrogen-guest detonation reaction at different high temperatures are necessary.
ReaxFF-MD [25,27,28] can conduct in-depth and detailed research on the pyrolysis mechanism of host-guest explosive at the microscopic scale and find how guest molecules participate in the pyrolysis reaction of the guest is an important factor affecting the energy release and detonation performance of the host explosives. In this study, we investigated the initial reaction of CL-20/CO 2 , CL-20/N 2 O, CL-20/NH 2 OH and compared with the pure CL-20/H 2 O at various temperatures (2500, 2750, 3000, 3250, and 3500 K) by ReaxFFlg reactive MD simulations (MD/ReaxFF-lg). The initial reaction paths, the change of generated/destroyed chemical bond numbers, the main product compositions, kinetic parameters in the different stages were analyzed. The mechanism for the improvement of the explosive energy and stability by incorporation of CO 2 , N 2 O and NH 2 OH is also discussed.  Figure 2. All the PE of CL-20/guest are much larger than that of CL-20/H 2 O. It demonstrates that the addition of small guest molecules significantly increases the energy levels of CL-20 just shown in Figure 3. All the systems exhibit an initial rise in the PE curves at different temperatures which correspond to the endothermic reaction stage. When PE is maximized, the value thereafter decreases, signifying that the reaction becomes exothermic. The maximum value of PE increases in the order incorporation of H 2 O < NH 2 OH < CO 2 < N 2 O at different temperatures, and the heat release is also increased. That is, the incorporation of guests may have remarkable influence on heat release during the reaction. At a relatively low temperature of 2500 K, the PE curve is smooth. However, when the temperature increases to 2750 K, the PE curve is a little bit steeper. As the temperature increases to, much higher, 3000 K and 3500 K, the PE curve changes little. That is, no obvious heat release occurs during the reaction for much higher temperatures. With the increase of temperature, the PE is observed to be close to equilibrium for much shorter time. Therefore, the higher the temperature, the earlier the complete reaction.

Results and Discussion
The evolution of potential energy (PE) of (a) CL-20/H 2 O, (b) CL-20/CO 2 , (c) CL-20/N 2 O, (d) CL-20/NH 2 OH system with time at different temperatures is shown in Figure 4. The trend of PE curves for CL-20/H 2 O and CL-20/N 2 O is very similar. The trend of PE curves for CL-20/CO 2 and CL-20/NH 2 OH is very similar. That is, the incorporation of N 2 O may have the same influence on heat release with H 2 O. However, the incorporation of CO 2 may have the same influence on heat release with NH 2 OH.   Figure  4. The trend of PE curves for CL-20/H2O and CL-20/N2O is very similar. The trend of PE curves for CL-20/CO2 and CL-20/NH2OH is very similar. That is, the incorporation of N2O   may have the same influence on heat release with H2O. However, the incorporation o CO2 may have the same influence on heat release with NH2OH.  Table 1 shows the initial reaction paths of host-guest molecules and their occurrenc frequency for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH at four high tempera tures. There are two main initial decomposition reactions of host CL-20 molecule C6H6O12N12 → C6H6O10N11 + NO2 and C6H6O12N12 → C6H5O12N12 + H. The frequency o C6H6O12N12 → C6H6O10N11 + NO2 is much more than that of C6H6O12N12 → C6H5O12N12 + H As the increase with the temperatures, the frequency of both main initial decomposition reaction improves for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH. A small part o H2O, N2O, NH2OH is broken to smaller pieces except for CO2 with no decomposition. A the same high temperature, the frequency of both main initial decomposition reactions i not significantly different for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH. I demonstrates that different guests have little influence on the initial decomposition paths There are three stages for the evolution of the thermal decomposition of CL-20/nitrogenguest. Firstly, the initial decomposition stage is characterized by rate constant k 1 and activation energy E a1. Then, the intermediate decomposition stage is characterized by rate constant k 2 and activation energy E a2. Finally, the final product evolution stage is characterized by rate constant k 3 and activation energy E a3 .

Initial Reaction Path of CL-20/Nitrogen-Guest
During the initial decomposition stage, the reaction rate was calculated by the change of the number of CL-20 molecules. The decay of the number of CL-20 molecules with time follows first-order decay exponential function [29]: where N 0 is the initial number of CL-20 molecules, t 0 is the time when CL-20 started to decompose, and k 1 is the initial decomposition stage rate constant ( Table 2). The logarithm of k 1 plotted against the inverse temperature (1/T) at 2500, 2750, 3000, 3250, and 3500 K is shown in Figure 5. The logarithm of k1 plotted against the inverse temperature (1/T) at 2500, 2750, 3000, 3250, and 3500 K is shown in Figure 5.  In addition, k1 of CL-20/N2O system is significantly larger than that of CL-20/H2O at high temperatures. This indicates that N2O significantly accelerates the reaction rate in the initial decomposition stage at high temperatures. k1 of CL-20/NH2OH system is significantly larger than that of CL-20/H2O at relatively higher temperatures (3500 K, 3250 K, 3000 K, 2750 K). As the temperature decreased to 2500 K, the difference between k1 of the CL-20/NH2OH and CL-20/N2O systems almost disappears. This indicates that NH2OH significantly accelerates the reaction rate in the initial decomposition stage at relatively higher temperature. k1 of CL-20/CO2 system is much complex. At higher temperatures, k1 In addition, k 1 of CL-20/N 2 O system is significantly larger than that of CL-20/H 2 O at high temperatures. This indicates that N 2 O significantly accelerates the reaction rate in the initial decomposition stage at high temperatures. k 1 of CL-20/NH 2 OH system is significantly larger than that of CL-20/H 2 O at relatively higher temperatures (3500 K, 3250 K, 3000 K, 2750 K). As the temperature decreased to 2500 K, the difference between k 1 of the CL-20/NH 2 OH and CL-20/N 2 O systems almost disappears. This indicates that NH 2 OH significantly accelerates the reaction rate in the initial decomposition stage at relatively higher temperature. k 1 of CL-20/CO 2 system is much complex. At higher temperatures, k 1 of CL-20/CO 2 is much larger than that of CL-20/H 2 O. However, k 1 of CL-20/CO 2 is much smaller than that of CL-20/H 2 O at relatively lower temperatures. This indicates the temperature has significant influence on the initial decomposition rate for CL-20/CO 2 . Figure 6 shows the evolution curves of the main intermediate products and host-guest molecules at different temperatures. For CL-20/H 2 O at 2500 K, the number curve of host CL-20 fluctuates slightly, but the overall level remains horizontal before 0.5 ps. The NO 2 fragments appears immediately at about 0ps. However, the number curve of guest H 2 O fluctuates slightly, but the overall level remains horizontal before 0.9 ps. This demonstrates that the initial decomposition of CL-20/H 2 O may have broken the C-NO 2 bonds of host CL-20 to form NO 2 . During 0.5 ps~1 ps, the number of host CL-20 decreases sharply and disappears at 1ps, while the number of NO 2 fragments increases rapidly. During 0.9 ps~1 ps, the number of guest H 2 O decreases, while the number of guest H 2 O reaches the minimum value. It demonstrates that guest H 2 O begins to participate the decomposition reaction deeply. The results of the trend are consistent with those of PE before 1ps for endothermic decomposition stage. During 0.5 ps~1 ps, the number of guest NO 2 increases sharply, while the number of guest NO 2 reaches the maximum value. Due to the participation of H 2 O, the pyrolysis products begin to diversify. The NO 3 and NO fragments begin to appear. All the curves for NO 3 and NO fragments are similarity at the high temperatures. However, the amount of NO 3 and NO fragments would improve as the increase of temperatures. As the temperature increased, the variation curves of the main intermediate produces and host-guest molecules for CL-20/H 2 O remains the same approximately. However, the reaction rates (k 2 ) are significantly different. The influence of high temperature on k 2 will be analyzed in the following section.

Intermediate Decomposition Stage 2.3.1. Effect of Nitrogen-Guest on the Main Intermediate Products
2.3.1. Effect of Nitrogen-Guest on the Main Intermediate Products Figure 6 shows the evolution curves of the main intermediate products and guest molecules at different temperatures. For CL-20/H2O at 2500 K, the number cur host CL-20 fluctuates slightly, but the overall level remains horizontal before 0.5 ps NO2 fragments appears immediately at about 0ps. However, the number curve of H2O fluctuates slightly, but the overall level remains horizontal before 0.9 ps. This de strates that the initial decomposition of CL-20/H2O may have broken the C-NO2 bon host CL-20 to form NO2. During 0.5 ps~1 ps, the number of host CL-20 decreases sh and disappears at 1ps, while the number of NO2 fragments increases rapidly. Durin ps~1 ps, the number of guest H2O decreases, while the number of guest H2O reache minimum value. It demonstrates that guest H2O begins to participate the decompos reaction deeply. The results of the trend are consistent with those of PE before 1p endothermic decomposition stage. During 0.5 ps~1 ps, the number of guest NO2 incr sharply, while the number of guest NO2 reaches the maximum value. Due to the pa pation of H2O, the pyrolysis products begin to diversify. The NO3 and NO fragm begin to appear. All the curves for NO3 and NO fragments are similarity at the high peratures. However, the amount of NO3 and NO fragments would improve as the inc of temperatures. As the temperature increased, the variation curves of the main inte diate produces and host-guest molecules for CL-20/H2O remains the same approxim However, the reaction rates (k2) are significantly different. The influence of high tem ture on k2 will be analyzed in the following section.
For CL-20/CO2, CL-20/N2O, CL-20/NH2OH at high temperatures, the evolution dency of the main intermediate products and host molecules is similar with that fo 20/H2O at 2500 K. The variation curves of the guest are quite different.   For CL-20/CO 2 , CL-20/N 2 O, CL-20/NH 2 OH at high temperatures, the evolution tendency of the main intermediate products and host molecules is similar with that for CL-20/H 2 O at 2500 K. The variation curves of the guest are quite different. Figures 7-9 show the evolution curves of the host and guest molecules for CL-20/H 2 O, CL-20/CO 2 , CL-20/N 2 O, CL-20/NH 2 OH at 2500 K, 3000 K, 3500 K. All the host CL-20 variation tendency are similarity. The influence of different guest on k 2 is not significant at 2500 K. With higher temperature, the k 2 significantly larger. The guest H 2 O and CO 2 , they are the main decomposition products. The variation tendency can divide into four stages: firstly, the tendency of guest is a level for little changeable. Secondly, it decreases for guest decomposition quickly. Then, it increases quickly as the main products. Finally, it reaches horizontal for the completely decomposition. There are two differences for the variation tendency: the first is that H 2 O and CO 2 start to decrease at different times. The longer time to stay at the first stage for CO 2 shows that CO 2 may be more stability than H 2 O, and the second is that the minimum values of H 2 O and CO 2 are different. It a greater intensity in pyrolysis reaction for H 2 O than that of CO 2 . As the increase of temperatures, the shorter time to stay the first stage and the smaller minimum for CO 2 and H 2 O. It displays that the more intense in pyrolysis reaction at higher temperatures. For N 2 O, NH 2 OH just as the role for guest, the variation tendency slowly decreases and then sharply decreases until disappears. However, the reaction rates (k 2 ) for CO 2  it reaches horizontal for the completely decomposition. There are two differences for the variation tendency: the first is that H2O and CO2 start to decrease at different times. The longer time to stay at the first stage for CO2 shows that CO2 may be more stability than H2O, and the second is that the minimum values of H2O and CO2 are different. It a greater intensity in pyrolysis reaction for H2O than that of CO2. As the increase of temperatures, the shorter time to stay the first stage and the smaller minimum for CO2 and H2O. It displays that the more intense in pyrolysis reaction at higher temperatures. For N2O, NH2OH just as the role for guest, the variation tendency slowly decreases and then sharply decreases until disappears. However, the reaction rates (k2) for CO2, N2O, NH2OH are significantly different at different temperatures.  it reaches horizontal for the completely decomposition. There are two differences for the variation tendency: the first is that H2O and CO2 start to decrease at different times. The longer time to stay at the first stage for CO2 shows that CO2 may be more stability than H2O, and the second is that the minimum values of H2O and CO2 are different. It a greater intensity in pyrolysis reaction for H2O than that of CO2. As the increase of temperatures, the shorter time to stay the first stage and the smaller minimum for CO2 and H2O. It displays that the more intense in pyrolysis reaction at higher temperatures. For N2O, NH2OH just as the role for guest, the variation tendency slowly decreases and then sharply decreases until disappears. However, the reaction rates (k2) for CO2, N2O, NH2OH are significantly different at different temperatures.

Effect of Nitrogen-Guest on the k2
After the PE reached the maximum value, the intermediate exothermic decomposition indicates the chemical reaction stage. The intermediate decomposition stage rate constant k2 can be obtained by fitting the PE curves with a first order decay exponential function [30]: is the potential energy value at time t, U∞ is the asymptotic value of PE, ΔUexo is the reaction heat, and its size is the difference between the maximum potential energy Umax and U∞.
The chemical reaction rate constants obtained by fitting equation at different temperatures are shown in Table 3. The value of ΔUexo has little change with a gradually increase of U∞ and k2 as temperature increases. This indicates that temperature has a limited effect on the exothermic reaction [31].

Effect of Nitrogen-Guest on the k 2
After the PE reached the maximum value, the intermediate exothermic decomposition indicates the chemical reaction stage. The intermediate decomposition stage rate constant k 2 can be obtained by fitting the PE curves with a first order decay exponential function [30]: is the potential energy value at time t, U ∞ is the asymptotic value of PE, ∆U exo is the reaction heat, and its size is the difference between the maximum potential energy U max and U ∞.
The chemical reaction rate constants obtained by fitting equation at different temperatures are shown in Table 3. The value of ∆U exo has little change with a gradually increase of U ∞ and k 2 as temperature increases. This indicates that temperature has a limited effect on the exothermic reaction [31]. The logarithm of k 2 plotted against the inverse temperature (1/T) at 2500, 2750, 3000, 3250, and 3500 K is shown in Figure 10. The logarithm of k2 plotted against the inverse temperature (1/T) at 2500, 2750, 3000, 3250, and 3500 K is shown in Figure 10.  This negative activation of entropy is consistent with the TST for multimolecular reactions, suggesting that the reaction involves a multimolecular transition state [32]. The decrease of entropy at the transition state because of the embedding of nitrogen-containing guests.
In addition, k2 of CL-20/CO2 system is significantly smaller than that of CL-20/H2O at high temperatures. This indicates that CO2 significantly inhibits the reaction in the intermediate decomposition stage at high temperatures. k2 of CL-20/N2O system is significantly smaller than that of CL-20/H2O at relatively lower temperatures (2500 K and 2750 K). As the temperature increased to 3500 K, the difference between k2 of the CL-20/H2O and CL- This negative activation of entropy is consistent with the TST for multimolecular reactions, suggesting that the reaction involves a multimolecular transition state [32]. The decrease of entropy at the transition state because of the embedding of nitrogen-containing guests.
In addition, k 2 of CL-20/CO 2 system is significantly smaller than that of CL-20/H 2 O at high temperatures. This indicates that CO 2 significantly inhibits the reaction in the intermediate decomposition stage at high temperatures. k 2 of CL-20/N 2 O system is significantly smaller than that of CL-20/H 2 O at relatively lower temperatures (2500 K and 2750 K). As the temperature increased to 3500 K, the difference between k 2 of the CL-20/H 2 O and CL-20/N 2 O systems almost disappears. This indicates that N 2 O significantly restrains the reaction in the intermediate decomposition stage at relatively low temperature. With increasing temperature, N 2 O has increasingly less effect on the reaction rate. However, k 1 of CL-20/N 2 O indicates that N 2 O significantly accelerates the reaction in the initial decomposition stage at high temperatures. The conclusion is contrary to that for ICM-102/HNO 3 [27]. This maybe caused by the hydrogen content for nitrogen-guest. k 2 of CL-20/H 2 O and CL-20/NH 2 OH systems are little difference at high temperatures, NH 2 OH has little effect on the reaction rate at high temperatures. The opposite effect of CL-20/H 2 O 2 [33] maybe due to the difference of hydrogen content in the guest. The influence of CO 2 and N 2 O on the decomposition reaction of host explosive may be the little interaction between CO 2 , N 2 O and CL-20 [27]. However, the influence of H 2 O 2 on the decomposition reaction of host explosive may be the significant interaction between H 2 O 2 and CL-20 [27].

Effect of Nitrogen-Guest on the Final Products
To clarify the effect of CO 2 , N 2 O and NH 2 OH molecules on the main products, the population of CO 2 , N 2 , H 2 O after the complete decomposition reaction for CL-20/H 2 O, CL-20/CO 2 , CL-20/N 2 O, CL-20/NH 2 OH at 2500 K, 3000 K and 3500 K are shown at  on the reaction rate at high temperatures. The opposite effect of CL-20/H2O2 [33] maybe due to the difference of hydrogen content in the guest. The influence of CO2 and N2O on the decomposition reaction of host explosive may be the little interaction between CO2, N2O and CL-20 [27]. However, the influence of H2O2 on the decomposition reaction of host explosive may be the significant interaction between H2O2 and CL-20 [27].

Effect of Nitrogen-Guest on the Final Products
To clarify the effect of CO2, N2O and NH2OH molecules on the main products, the population of CO2, N2, H2O after the complete decomposition reaction for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH at 2500 K, 3000 K and 3500 K are shown at Figures 11-13.   the decomposition reaction of host explosive may be the little interaction between CO2, N2O and CL-20 [27]. However, the influence of H2O2 on the decomposition reaction of host explosive may be the significant interaction between H2O2 and CL-20 [27].

Effect of Nitrogen-Guest on the Final Products
To clarify the effect of CO2, N2O and NH2OH molecules on the main products, the population of CO2, N2, H2O after the complete decomposition reaction for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH at 2500 K, 3000 K and 3500 K are shown at Figures 11-13.   The population of CO2 for CL-20/NH2OH and CL-20/H2O are nearly equivalent at high temperatures. The population of CO2 for CL-20/N2O is the lowest, while the popula- The population of CO 2 for CL-20/NH 2 OH and CL-20/H 2 O are nearly equivalent at high temperatures. The population of CO 2 for CL-20/N 2 O is the lowest, while the population of CO 2 for CL-20/H 2 O is largest at 2500 K. As the temperature increase, the more population of CO 2 for CL-20/N 2 O grows acutely. However, the population of CO 2 for CL-20/CO 2 , CL-20/NH 2 OH decrease acutely. It demonstrates that the CO 2 -produced mechanism for a N 2 O guest is different with CO 2 and NH 2 OH guests. The populations of N 2 for CL-20/NH 2 OH and CL-20/H 2 O are nearly equivalent at high temperatures. As the temperature increases, the more the population of CO 2 for CL-20/CO 2 , CL-20/N 2 O grows acutely. It demonstrates that the N 2 -produced mechanism for NH 2  The final products of thermal decomposition of CL-20/guest are N 2 , CO 2 and H 2 O. The formation rates k 3 can be obtained by fitting the variation trend of the final products with the exponential function [34]:  Figure 15. For the CL-20/CO system, the k3 value of CO2 is slightly higher than that for CL-20/H2O, CL-20/N2O, CL-

Discussion
We have performed MD/ReaxFF-lg simulations to investigate the thermal decomposition reaction of the CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH systems at different temperatures. In this work, guest is not only enhanced the safety but also improved its detonation performance.
During the thermal decomposition reaction of CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH systems at different temperatures, the initial reaction path is not significantly influenced by the incorporation of CO2, N2O, NH2OH: C6H6O12N12 → C6H6O10N11 + NO2 and C6H6O12N12 → C6H5O12N12 + H. At the same high temperature, the frequencies of both two main initial decomposition reaction are not significantly different for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH. As for the increase with the temperatures, the frequency of both main initial decomposition reactions improve for CL-20/H2O, CL-20/CO2, CL-20/N2O, CL-20/NH2OH. The nitrogen guest can inhibit the trigger decomposition for the larger Ea1 and Ea2 values. Embedding N2O and NH2OH can significantly accelerate the reaction in the initial decomposition rates at high temperatures for the larger k1 at high temperatures. However, incorporation of CO2, higher temperature has a significant influence on the initial decomposition for the complex k1. Embedding CO2 and N2O significantly inhibits the reaction in the intermediate decomposition stage at high temperatures for the smaller k2 at high temperatures. Incorporation of NH2OH has little effect on the reaction rate at high temperatures, with a small difference of k2. All the k3 values of CO2, H2O and N2 are increased as the temperature improves. Guest CO2 restrains the formation of H2O and N2 molecules for the higher k3 value. Guest N2O restrains the formation of CO2 for the higher k3 value. Guest NH2OH accelerates the formation of H2O molecules for the higher k3 value. The influence of guest NH2OH and H2O, N2O and CO2 on decomposition products may be similar for the same amount products.
The results of this study revealed that the guest CO2, N2O and NH2OH played a cer-

Discussion
We have performed MD/ReaxFF-lg simulations to investigate the thermal decomposition reaction of the CL-20/H 2 O, CL-20/CO 2 , CL-20/N 2 O, CL-20/NH 2 OH systems at different temperatures. In this work, guest is not only enhanced the safety but also improved its detonation performance.
During The results of this study revealed that the guest CO 2 , N 2 O and NH 2 OH played a certain inhibitory role during the early stages of the host CL-20 thermal decomposition reaction. The study provided a theoretical basis for the synthesis of new energetic materials with host-guest inclusion strategy.

Computational Methods
The  First, the canonical ensemble (NVT) and the Berendsen thermostat were applied to the molecular dynamics (MD) simulation with a total time of 10 ps at 1 K, which further relaxed the α-CL-20/guest supercell. Then, ReaxFF-lg isobaric-isothermal MD (NPT-MD) simulations were performed for 5 ps at 300 K controlled by the Berendsen thermostat based on the relax supercell. Finally, the target temperatures (2500, 2750, 3000, 3250, and 3500 K) are all direct heat from 300 K with NVT-MD simulations. The five different high temperatures are selected to accurately calculate the reaction rate constant and energy barrier. The damping constant is set to 0.25 fs. Komeiji demonstrated that 0.25 fs is enough for calculation accuracy of bonds and angles in molecular dynamics simulations. NVT-MD simulations of the supercell system with the Berendsen thermostat were performed until the potential energy (PE) stabilized. An analysis of the fragments was performed with a 0.3 bond order cutoff value for each atom pair to identify the chemical species [35,36]. The information of the dynamic trajectory was recorded every 20 fs, which was used to analyze the evolution details of α-CL-20/guest in the pyrolysis process.
To verify the suitability of the ReaxFF-lg force field for the CL-20/guest system, we compared the lattice parameters and density of relaxed CL-20/guest at 298 K and 0 Pa with the initial structure from the CCDC ( Table 4). The cell parameters and density of relaxed CL-20/guest calculated by MD/ReaxFF-lg agreed well with the initial structure values for the error value < 5%. This preliminarily indicated that ReaxFF-lg can describe the decomposition of CL-20/guest system. First, the canonical ensemble (NVT) and the Berendsen thermostat were applied to the molecular dynamics (MD) simulation with a total time of 10 ps at 1 K, which further relaxed the α-CL-20/guest supercell. Then, ReaxFF-lg isobaric-isothermal MD (NPT-MD) simulations were performed for 5 ps at 300 K controlled by the Berendsen thermostat based on the relax supercell. Finally, the target temperatures (2500, 2750, 3000, 3250, and 3500 K) are all direct heat from 300 K with NVT-MD simulations. The five different high temperatures are selected to accurately calculate the reaction rate constant and energy barrier. The damping constant is set to 0.25 fs. Komeiji demonstrated that 0.25 fs is enough for calculation accuracy of bonds and angles in molecular dynamics simulations. NVT-MD simulations of the supercell system with the Berendsen thermostat were performed until the potential energy (PE) stabilized. An analysis of the fragments was performed with a 0.3 bond order cutoff value for each atom pair to identify the chemical species [35,36]. The information of the dynamic trajectory was recorded every 20 fs, which was used to analyze the evolution details of α-CL-20/guest in the pyrolysis process.
To verify the suitability of the ReaxFF-lg force field for the CL-20/guest system, we compared the lattice parameters and density of relaxed CL-20/guest at 298 K and 0 Pa with the initial structure from the CCDC ( Table 4). The cell parameters and density of relaxed CL-20/guest calculated by MD/ReaxFF-lg agreed well with the initial structure values for the error value < 5%. This preliminarily indicated that ReaxFF-lg can describe the decomposition of CL-20/guest system.