COMPUTATIONAL CHEMISTRY AND MOLECULAR SIMULATION CURRENT SCIENCE, VOL. 106, NO. 9, 10 MAY 2014 1219 *For correspondence. (e-mail: gvsp@uohyd.ernet.in) Structural properties of solid energetic materials: a van der Waals density functional study G. Vaitheeswaran*, K. Ramesh Babu, N. Yedukondalu and S. Appalakondaiah Advanced Centre of Research in High Energy Materials, University of Hyderabad, Prof. C. R. Rao Road, Gachibowli, Hyderabad 500 046, India In the present work we have focused our attention towards the complete description of structural proper- ties of energetic solids, namely inorganic azides, secondary explosives and oxidizers through density functional theory-based calculations. We find large deviations in structural parameters calculated with the standard exchange-correlation functional such as local density approximation and generalized gradient approximation (GGA). On the other hand, dispersion- corrected density functional of (GGA + G06) describes the crystal structure of the energetic solids with good accuracy. This leads to the fact that the dispersion- corrected density functionals are essential to describe the crystal structure and thereby the related physical and chemical properties of the energetic solids. Keywords: Crystal structure, density functional theory, dispersion interactions, energetic solids. Introduction SOLID energetic materials find numerous applications as explosives and fuels in defence, space and civilian sec- tors 1 . The performance characteristics of energetic materials can be assessed by looking at their sensitivity towards external stimuli and other properties such as detonation pressure and detonation velocity which solely depend upon the crystal density and crystal structure 2 . Thus for any energetic system, the accurate crystal struc- ture description is mandatory to assess the fundamental properties related to its performance. In general, the cry- stal structure of energetic materials consists of molecular units composed of a large number of atoms. These mole- cular units bind through weak dispersive or van der Waals interactions 3,4 . As the energetic systems contain a large number of atoms and complex chemical bonding, simulation of these kinds of molecular solids is a con- tinuous challenge. Density functional theory (DFT) is a successful tool in simulating and predicting the physical and chemical properties of a wide spectrum of materials 5,6 . In many cases the results obtained through DFT calculations are in reasonable agreement with experimental data. However, most of the energetic materials have complicated crystal structures with weak intermolecular interactions and hence the investigation of different physical and chemical properties of energetic materials through DFT is really a challenging task 7 . Byrd et al. 8 have applied DFT to pre- dict the structural properties of various energetic materi- als at ambient conditions. However, the predicted lattice parameters had large errors relative to the experiments. This is due to the fact that the usual DFT-based exchange-correlation functionals obtained within the local density approximation (LDA) and generalized gra- dient approximation (GGA) are not accurate enough to describe the systems having very small electronic overlap between the constituent atoms. Nevertheless, DFT can predict the lattice parameters of the energetic materials that are in close agreement with experiments provided the exchange-correlation functionals are corrected to describe the weak intermolecular interactions 9–14 . In this present study, we have considered three kinds of energetic materials, namely inorganic azides NaN 3 , Ca(N 3 ) 2 , Sr(N 3 ) 2 ; secondary explosives nitromethane (CH 3 NO 2 ), HMX (C 4 H 8 N 8 O 8 ), FOX-7 (C 2 H 4 N 4 O 4 ) and oxidizers LiNO 3 , NaNO 3 and KNO 3 . The motivation behind choosing these three categories is the following: inorganic azides can be used as gas generators and also they are potential precursors to synthesize polymeric nitrogen, which is considered to be an ultimate green high-energy density material 15 . On the other hand, the secondary explosives are highly energetic systems and are used in war heads 16 . Oxidizers are oxygen-rich solids that decompose at moderate-to-high temperatures liberat- ing oxygen gas 17 . The concentration of oxygen within an explosive or oxidizer is represented by a term known as ‘oxygen balance’, which is an important parameter for identifying its potential as an explosive or oxidizer. In general, the secondary explosives like HMX (–21.62%), and RDX (C 3 H 6 N 6 O 6 ) (–21.6%) have oxygen deficiency and hence it is necessary to bind an oxidizer to the explo- sive 18 . The alkali metal nitrates LiNO 3 , NaNO 3 and KNO 3 can act as oxidizers with positive oxygen balance 58.1%, 47.1% and 39.6% respectively. They find applications as fertilizers, pyrotechnical compositions, and industrial explosives and in black powder 18 . To the best of our knowledge the complete description of the crystal structure