Zero-temperature equation of state of solid 4He at low and high pressures

We study the zero-temperature equation of state (EOS) of solid (4)He in the hexagonal closed packed (hcp) phase over the 0-57 GPa pressure range by means of the diffusion Monte Carlo (DMC) method and the semi-empirical Aziz pair potential HFD-B(HE). In the low pressure regime (P similar to 0-1 GPa) we assess excellent agreement with experiments and we give an accurate description of the atomic kinetic energy, Lindemann ratio and Debye temperature over a wide range of molar volumes (22-6 cm(3) mol(-1)). However, on moving to higher pressures our calculated P-V curve presents an increasingly steeper slope, which ultimately provides differences within similar to 40% with respect to measurements. In order to account for many-body interactions arising in the crystal with compression which are not reproduced by our model, we perform additional electronic density functional theory (DFT) calculations for correcting the computed DMC energies in a perturbative way. We explore both generalized gradient and local density approximations (GGA and LDA, respectively) for the electronic exchange-correlation potential. By proceeding in this manner, we show that discrepancies with respect to high pressure data are reduced to 5-10% with few extra computational costs. Further comparison between our calculated EOSs and ab initio curves deduced for the perfect crystal and corrected for the zero-point motion of the atoms enforces the reliability of our approach.