Solid hydrogen and deuterium. II. Pressure and compressibility calculated by a lowest-order constrained-variation method

The pressure and the compressibility of solid H₂ and D₂ are obtained from ground-state energies calculated by means of a modified variational lowest-order constrained-variation (LOCV) method. Both fcc and hcp structures are considered, but results are given for the fcc structure only. The pressure and the compressibility are calculated or estimated from the dependence of the ground-state energy on density or molar volume, generally in a density region of 0.65Σ⁻³ to 1.3Σ⁻³, corresponding to a molar volume of 12–24 cm³/mole, where Σ = 2.958 å, and the calculations are done for five different two-body potentials. Theoretical results for the pressure are 340–460 atm for solid H₂ at a particle density of 0.82Σ⁻³ or a molar volume of 19 cm³/mole, and 370–490 atm for solid ⁴He at a particle density of 0.92Σ⁻³ or a molar volume of 17 cm³/mole. The corresponding experimental results are 650 and 700 atm, respectively. Theoretical results for the compressibility are 210 × 10⁻⁶ to 260 × 10⁻⁶ atm⁻¹ for solid H₂ at a particle density of 0.82Σ⁻³ or a molar volume of 19 cm³/mole, and 150 × 10⁻⁶ to 180 × 10⁻⁶ atm⁻¹ for solid D₂ at a particle density of 0.92Σ⁻³ or a molar volume of 17 cm³/mole. The corresponding experimental results are 180 × 10⁻⁶ and 140 × 10⁻⁶ atm⁻¹, respectively. The agreement with experimental results is better for higher densities.