Density condensed phase calculations

That classical calculation may be a density functional theory (DFT) ab initio simulation. An ab initio treatment may be important to handle charge redistribution effects in the condensed phase. 

Lewis et al.106 calculated four possible decomposition pathways of the ot-HMX polymorph N-N02 bond dissociation, HONO elimination, C-N bond scission, and concerted ring fission. Based on energetics, it was determined that N-N02 dissociation was the initial mechanism of decomposition in the gas phase, whereas they proposed HONO elimination and C-N bond scission to be favorable in the condensed phase. The more recent study of Chakraborty et al.42 using density functional theory (DFT), reported detailed decomposition pathways of p-HMX, which is the stable polymorph at room temperature. It was concluded that consecutive HONO elimination (4 HONO) and subsequent decomposition into HCN, OH, and NO are the most energetically favorable pathways in the gas phase. The results also showed that the formation of CH20 and N20 could occur preferably from secondary decomposition of methylenenitramine. 

Fig. 5 shows that the mean free path of low-energy carbon ions is less than that of a water molecular diameter. Calculations based on such short mean free paths may predict two energy loss events within the same molecule. However, as discussed elsewhere in this book, the double ionization cross section is much lower than predicted by these results. The problem is that cross sections are normally based on a single isolated collision at gaseous density. Extrapolation to the condensed phase can lead to unrealistic predictions. 

The phase state of the medium in which the studied processes occur is very important in such calculations. For quite a long time all authors studying the primary processes in the condensed phase have used the traditional model of a medium as an ensemble of independent molecules with correction only for the greater density.17 In the recent years there has been considerable process toward a more realistic account of the specific features of condensed media.14 18,19... 

For gases, n = e 1 is an excellent approximation. The easiest approach to condensed phases maintains this approximation, where calculations of the molecular first-order and response properties are performed for the isolated molecule, while accounting for the effect of intermolecular interactions through the number density N = Aa/Vm, and therefore by taking appropriate values of Vm. This rough, often at best qualitative, approach is somewhat relaxed by employing expansions of the birefringence constant with the density, that is in inverse powers of Vm. This introduces the appropriate virial coefficients [15,16]... 

Other flexible molecular models of nitromethane were developed by Politzer et al. [131,132]. In these, parameters for classical force fields that describe intramolecular and intermolecular motion are adjusted at intervals during a condensed phase molecular dynamics simulation until experimental properties are reproduced. In their first study, these authors used quantum-mechanically calculated force constants for an isolated nitromethane molecule for the intramolecular interaction terms. Coulombic interactions were treated using partial charges centered on the nuclei of the atoms, and determined from fitting to the quantum mechanical electrostatic potential surrounding the molecule. After an equilibration trajectory in which the final temperature had been scaled to the desired value (300 K), a cluster of nine molecules was selected for a density function calculation from which... 

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