A New Method for Calculation of Elastic properties of Anisotropic Material by Constant Pressure Molecular Dynamics

Abstract

A new method based on constant pressure molecular dynamics (MD) with the software package Materials Studio (MS) was developed to calculate the anisotropic elastic properties of 1,3,5-tri-amino-2,4,6-tri-nitrobenzene (TATB) which is a typical and widely studied explosive molecular and its single crystal is a typical triclinic lattice. Key points of the method are introduced. Firstly, a P1 periodic super cell of TATB which ab plane was designated as (010) planar of the crystal was modeled. After several fixing and relaxing steps, the cell was pre-equilibrated for 500 ps and followed a 100 ps MD with trajectory at 298 K in NVT ensemble. The averaged isotropic elastic properties were calculated by MS analysis module of elastic properties (static). Secondly, APT ensemble was chosen and different stresses in six different directions were added through many tries to keep the cell parameters fluctuating around those values in NVT ensemble while performing constant pressure MD. The MS averaged isotropic elastic properties as well as elastic constant matrix of the well equilibrated NPT system were then obtained. Thirdly, different magnitude of compressive or tensile stress was applied to the cell in one direction and the equilibrated box lengths were averaged statistically to calculate the strains in three directions. Anisotropic tensile moduli as well as Poisson's ratios were then obtained via the strain-stress profile and the averaged values over those in three directions were given. Finally, the average values were compared with those in NVT or NPT ensemble to validate the titled method. Deviation within less than 20% revealed that the method can be certainly applied to the study on elastic properties of anisotropic materials.