VIBRATIONAL AND THERMODYNAMIC PROPERTIES OF 1,3,5-TRIAMINO-2,4,6-TRINITROBENZENE (TATB): COMPARISON OF EXCHANGE-CORRELATION FUNCTIONALS IN DENSITY FUNCTIONAL THEORY

Vibrational and thermodynamic properties of TATB have been investigated within the quasiharmonic approximation and density functional theory using three exchange-correlation functionals: local density approximation (LDA), generalized gradient approximation (GGA), and GGA with an empirical van der Waals correction (GGA + vdW). We find that GGA provides a reasonable description of only the heat capacity and thermal expansion, while it fails to reproduce the experimental bulk modulus and volume. Van der Waals correction improves the lattice constants, volume, and bulk modulus, but it fails badly in describing thermal expansion and heat capacity. In contrast, LDA accurately describes all the thermodynamic properties of TATB considered here. For example, the equilibrium volume calculated with LDA is only 4.6% smaller than the experimental value after including vibrational contributions. It is therefore essential to include phonon contributions when comparing the calculated volume with experimental data at ambient conditions. We show that an accurate equation of state of TATB is obtained by simply multiplying the volume calculated with LDA by a factor of 1.046, because LDA predicts the bulk modulus well in the entire pressure range. Therefore, LDA is a satisfactory exchange-correlation functional for TATB because only LDA correctly predicts the volume dependence of vibrational frequencies. All calculations exhibit an abrupt change of the compressibility at a critical pressure, P-c similar to 0.5-1.0 GPa. Below P-c, the volume reduction by pressure is mainly due to the lattice contraction along the c axis, whereas above P-c the lattice contracts significantly along all three axes.