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Published October 13, 2011 | Supplemental Material
Journal Article Open

ReaxFF-lg: Correction of the ReaxFF Reactive Force Field for London Dispersion, with Applications to the Equations of State for Energetic Materials

Abstract

The practical levels of density functional theory (DFT) for solids (LDA, PBE, PW91, B3LYP) are well-known not to account adequately for the London dispersion (van der Waals attraction) so important in molecular solids, leading to equilibrium volumes for molecular crystals ∼10-15% too high. The ReaxFF reactive force field is based on fitting such DFT calculations and suffers from the same problem. In the paper we extend ReaxFF by adding a London dispersion term with a form such that it has low gradients (lg) at valence distances leaving the already optimized valence interactions intact but behaves as 1/R^6 for large distances. We derive here these lg corrections to ReaxFF based on the experimental crystal structure data for graphite, polyethylene (PE), carbon dioxide, and nitrogen and for energetic materials: hexahydro-1,3,5-trinitro- 1,3,5-s-triazine (RDX), pentaerythritol tetranitrate (PETN), 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), and nitromethane (NM). After this dispersion correction the average error of predicted equilibrium volumes decreases from 18.5 to 4.2% for the above systems. We find that the calculated crystal structures and equation of state with ReaxFF-lg are in good agreement with experimental results. In particular, we examined the phase transition between α-RDX and γ-RDX, finding that ReaxFF-lg leads to excellent agreement for both the pressure and volume of this transition occurring at ∼4.8 GPa and ∼2.18 g/cm^3 density from ReaxFF-lg vs 3.9 GPa and ∼2.21 g/cm^3 from experiment. We expect ReaxFF-lg to improve the descriptions of the phase diagrams for other energetic materials.

Additional Information

© 2011 American Chemical Society. Received: February 17, 2011. Revised: August 22, 2011. Publication Date (Web): September 2, 2011. Financial support from the National Science Foundation of China (No. 20473052), NSAF funding (No. 10676021), and the National Basic Research Program of China (Nos. 2003CB615804 and 2007CB209701) is gratefully acknowledged. We acknowledge the funding from the China Scholarship Council (No. 2009623057, Cliff Bedford). In addition, we acknowledge Funding ONR (N00014-09-1-0634), ARO-MURI (W911NF-08-1-0124, Ralph Anthenien), and ARL HPC (with help from Betsy Rice).

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