Ab initio -based intermolecular potential for normal hydrogen, orthohydrogen, and parahydrogen: Prediction of thermodynamic behavior

An ab initio-based intermolecular potential is reported for normal hydrogen, orthohydrogen, and parahydrogen. Molecular simulations for thermodynamic properties and calculations of the second virial coefficients demonstrate very good agreement with reference values for both the nuclear spin isomers of hydrogen. The thermodynamic behavior studied includes vapor–liquid equilibria, energy, enthalpy, heat capacities, isochoric pressure coefficient, isobaric thermal expansion coefficient, isothermal compressibility, Joule–Thomson coefficient, and speed of sound. The simulations extend from low (cryogenic) to ambient temperatures and pressures of up to 100 MPa. Quantum effects have an important influence on the behavior of these properties, which, in some cases, are not limited to the intermolecular potential contribution to the energy. That is, the enthalpy, heat capacities, Joule–Thomson coefficient, and speed of sound are also significantly affected by quantum effects on the kinetic energy. The simulation values may be more reliable than existing reference data in some instances. This is particularly the case for the thermodynamic properties of orthohydrogen that cannot be measured directly.