Zero-temperature equation of state of solid4He at low and high pressures

Abstract

We study the zero-temperature equation of state (EOS) of solid4He in the hexagonal closed packed (hcp) phase over the 0–57 GPa pressurerange by means of the diffusion Monte Carlo (DMC) method and thesemi-empirical Aziz pair potential HFD–B(HE). In the low pressure regime(P∼0–1 GPa) we assess excellent agreement with experiments and we give an accurate description of theatomic kinetic energy, Lindemann ratio and Debye temperature over a wide range of molar volumes(22–6 cm3 mol−1). However, on moving to higher pressures our calculatedP–V curve presents an increasingly steeper slope, which ultimately provides differences within∼40% with respect to measurements. In order to account for many-body interactionsarising in the crystal with compression which are not reproduced by our model,we perform additional electronic density functional theory (DFT) calculationsfor correcting the computed DMC energies in a perturbative way. We exploreboth generalized gradient and local density approximations (GGA and LDA,respectively) for the electronic exchange–correlation potential. By proceeding inthis manner, we show that discrepancies with respect to high pressure data arereduced to 5–10% with few extra computational costs. Further comparison betweenour calculated EOSs and ab initio curves deduced for the perfect crystal andcorrected for the zero-point motion of the atoms enforces the reliability of ourapproach.