Calculation of bulk modulus for highly anisotropic materials

Bulk modulus is usually obtained from fitting experimental or theoretical energy-volume data to an empirical functional form of the equation of state. Most theoretical studies generate the energy-volume data by uniformly changing the unit cells in total-energy calculations. This approach works well for isotropic systems such as structures with cubic symmetries; but for noncubic systems with high anisotropy, full structural relaxation has to be carried out at each volume to ensure correct extraction of bulk modulus. In this Letter, we present an approach that uses uniaxial compression and the volume derivative (of energy) definition of bulk modulus. First principles pseudopotential calculations are carried out with the generalized gradient approximation applied to more localized core electrons while the local density approximation applied to valence electrons to improve the accuracy of calculated structural data. We have applied this approach to elemental hexagonal graphite and binary hexagonal boron nitride and have obtained results in excellent agreement with experiment. We also have used this approach to predict the value of bulk modulus for a graphitic phase of a ternary BC2N structure. (C) 2004 Elsevier B.V. All rights reserved.