Further Monte Carlo studies of the rotation-libration transition in solid H₂ under pressure

The pressure-induced transition in solid para-H₂ from free rotation of molecules to libration around the three-fold axes of the fcc structure is calculated using a completely anisotropic interaction potential of the form % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeOvamaaBa% aaleaacaqGPbGaaeOAaaqabaGccqGH9aqpdaaeqaqaaiaaboeaaSqa% aiaab2gaaeqaniabggHiLdGcdaWgaaWcbaGaaeyBaaqabaGccaGGOa% GaaeOCamaaBaaaleaacaqGPbGaaeOAaaqabaGccaGGPaGaaeywamaa% BaaaleaacaqGYaGaaeyBaaqabaGccaGGOaGaeuyQdC1aaSbaaSqaai% aabMgaaeqaaOGaaiykaiaabMfadaWgaaWcbaGaaeOmaiaab2cacaqG% TbaabeaakiaacIcacqqHPoWvdaWgaaWcbaGaaeOAaaqabaGccaGGPa% aaaa!5132!\[{\text{V}}_{{\text{ij}}} = \sum\nolimits_{\text{m}} {\text{C}} _{\text{m}} ({\text{r}}_{{\text{ij}}} ){\text{Y}}_{{\text{2m}}} (\Omega _{\text{i}} ){\text{Y}}_{{\text{2 - m}}} (\Omega _{\text{j}} )\] including quadrupole-quadrupole and repulsive valence interactions. The rotational part of the Hamiltonian is assumed to be operating on a Jastrow-type wave function, including two variational parameters. The variational integrals are evaluated by a Monte Carlo procedure. The orientational order parameter in the low-density region increases very gradually with increasing density, until a sharp jump (indicating first-order) is observed at the transition density corresponding to R/R ₀ = 0.5775, where R ₀ is the nearest neighbor distance at zero pressure. This is 60% higher than the density obtained in previous work.