Refined phase coexistence line between graphite and diamond from density-functional theory and van der Waals correction

In the present paper, we provide the refined pressure-temperature phase coexistence line between graphite and diamond with van der Waals (vdW) energy corrected density-functional theory and quasiharmonic approximation. A systematic comparison of the generalized gradient approximation (GGA) results for the lattice-related properties, phonon dispersion curves, and thermodynamic properties of graphite and diamond with and without vdW correction is presented. It is shown that adding the vdW interactions over the GGA exchange-correlation energy is positively necessary to obtain the reliable phase coexistence line between graphite and diamond, as compared with the experimental data. The 0 K phase transition pressure calculated with the GGA+vdW formalism is 1.6 GPa, which is sufficiently close to the value of 1.4 GPa extrapolated from the experimental values. We find the flattening of the coexistence line at low temperatures below ~300K with GGA+vdW as well as GGA. The slope of the coexistence line calculated by the GGA+vdW approach is 2.5×10−3 GPa K−1 placed within the experimental value range.