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Van der Waals Density Functional Theory vdW-DFq for Semihard Materials

1
Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180, USA
2
School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, Hubei, China
3
Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109, USA
4
School of Aerospace Systems, University of Cincinnati, Cincinnati, OH 45221, USA
*
Author to whom correspondence should be addressed.
Crystals 2019, 9(5), 243; https://doi.org/10.3390/cryst9050243
Received: 9 April 2019 / Revised: 1 May 2019 / Accepted: 2 May 2019 / Published: 8 May 2019
(This article belongs to the Special Issue First-Principles Prediction of Structures and Properties in Crystals)
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Abstract

There are a large number of materials with mild stiffness, which are not as soft as tissues and not as strong as metals. These semihard materials include energetic materials, molecular crystals, layered materials, and van der Waals crystals. The integrity and mechanical stability are mainly determined by the interactions between instantaneously induced dipoles, the so called London dispersion force or van der Waals force. It is challenging to accurately model the structural and mechanical properties of these semihard materials in the frame of density functional theory where the non-local correlation functionals are not well known. Here, we propose a van der Waals density functional named vdW-DFq to accurately model the density and geometry of semihard materials. Using β -cyclotetramethylene tetranitramine as a prototype, we adjust the enhancement factor of the exchange energy functional with generalized gradient approximations. We find this method to be simple and robust over a wide tuning range when calibrating the functional on-demand with experimental data. With a calibrated value q = 1.05 , the proposed vdW-DFq method shows good performance in predicting the geometries of 11 common energetic material molecular crystals and three typical layered van der Waals crystals. This success could be attributed to the similar electronic charge density gradients, suggesting a wide use in modeling semihard materials. This method could be useful in developing non-empirical density functional theories for semihard and soft materials. View Full-Text
Keywords: density functional theory; van der Waals corrections; semihard materials; molecular crystals density functional theory; van der Waals corrections; semihard materials; molecular crystals
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Peng, Q.; Wang, G.; Liu, G.-R.; De, S. Van der Waals Density Functional Theory vdW-DFq for Semihard Materials. Crystals 2019, 9, 243.

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