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Ab initio calculation of intermolecular potential parameters for gaseous decomposition products of energetic materials

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Abstract

This document describes the results obtained using two methods for ab initio calculation of intermolecular potential parameters for gaseous decomposition products of energetic materials: a Multipole Expansion method, suitable for axially symmetric molecules, and a Monte Carlo method, which can be used to obtain temperature dependent average potential energy parameters for any molecule.

Executive Summary

The weapons systems of the Australian Defence Force rely on energetic materials for rocket and projectile propulsion (propellants) and terminal effects (explosives). In addition to research aimed at decreasing energetic material sensitivity for greater safety, improvements in performance are also being sought for increased range and thrust control in propellants and for enhanced terminal effects in explosives. It is necessary to model the decomposition of energetic materials (combustion for propellants and detonation for explosives) in order to gain a predictive capability for the performance of existing and potential energetic materials which may be utilised in future weapons systems. The modelling of the decomposition processes involves sophisticated numerical models, which require equations of state describing properties of the decomposition products such as pressure, volume and internal energy. These decomposition products are largely gases, which in the high pressure, high temperature regime present during the combustion or detonation, are highly non-ideal. The non-ideal behaviour of gases is largely determined by the interactions between pairs of gas molecules which can be described as an intermolecular potential. Consequently, if we can obtain accurate estimates of the intermolecular potentials present, we can accurately model the decomposition of the energetic materials, leading to accurate estimates of their performance and decomposition behaviour. Intermolecular potential parameters are generally obtained empirically by choosing them to be consistent with experimental results. However, if the intermolecular potentials can be obtained directly by computation, we can calculate their parameters independently of experiments, which can then be used to validate the results of experiments and also to give a predictive capability for areas where experiments have not, or can not, be conducted. Ab initio molecular orbital theory enables the calculation of molecular parameters, such as geometry and energy, using only approximations to the Schrodinger equation, without requiring any experimental data or assumptions from the system under study. The purpose of the current study was to attempt to obtain intermolecular potentials for gaseous products of energetic material decomposition for use in equations of state using only ab initio molecular orbital calculations1. It was found that these potentials can be calculated to a good degree of accuracy, giving valuable inputs for equations of state and allowing the subsequent calculation of the performance of energetic materials. 1 The work presented in this document was commenced while Dr Alex White was on Long Term Attachment to the Naval Surface Warfare Center, Indian Head, Maryland and subsequently completed after his return to DSTO Salisbury.

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