Quantum corrections to the kinetic energy and the <i>ab initio</i>-based prediction of the thermodynamic properties and vapor–liquid equilibria of hydrogen

The ability of ab initio-based intermolecular potentials to predict the vapor-liquid-equilibria (VLE) and thermodynamic properties of hydrogen is investigated via Monte Carlo simulation. The combination of a simplified ab initio atomic potential (SAAP) and first order Feynman-Hibbs (FH-1) interactions closely reproduces the VLE phase envelope, providing a good estimate of the critical point. The SAAP + FH-1 combination also improves the prediction of other thermodynamic properties. However, the accurate determination of enthalpy, heat capacity, isothermal compressibility, isochoric pressure coefficient, and isobaric thermal expansion coefficient requires the addition of a quantum correction to the kinetic energy (QCKE). The QCKE is a post-simulation contribution to the thermodynamic properties of quantum fluids and, as such, can be used to improve the accuracy of any predictions using an intermolecular potential. The addition of QCKE to the SAAP + FH-1 potential results in values for the thermodynamic properties that are close to reference data for hydrogen at temperatures greater than 40 K and pressures up to 100 MPa.