Richard J. Sadus

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.

Molecular simulations, using an accurate ab initio-based intermolecular potential, are reported, providing a comprehensive description of the thermodynamic behavior of tritium (T2) at both low (cryogenic) and ambient temperatures and at pressures up to 1 GPa. The study includes the second virial coefficient, vapor–liquid equilibria, energy, enthalpy, heat capacities, isochoric pressure coefficient, isobaric thermal expansion coefficient, isothermal compressibility, Joule–Thomson coefficient, and the speed of sound. The simulations demonstrate the important role of quantum effects on the behavior of these properties. Comparison with the corresponding properties of hydrogen (H2) indicates that there are often large property-dependent differences between the two species. The results confidently extend our knowledge of the thermodynamics of T2 beyond the very scarce amount of available experimental data.

Molecular simulation with an ab initio-based intermolecular potential is used to investigate the atomic-level interactions responsible for the thermodynamic properties and vapor–liquid-equilibria (VLE) of deuterium (D₂). Results are reported for pressures up to 100 MPa at both low (cryogenic) and ambient temperatures. The combination of a simplified ab initio atomic potential (SAAP) with Feynman–Hibbs first order (FH1) interactions closely reproduces the VLE phase envelope over a wide range of densities, resulting in good estimates of the critical properties. It also accurately reproduces the behavior of the second virial coefficient and pressure–temperature–volume properties. The analysis indicates that deuterium and hydrogen share the same intermolecular potential, i.e., SAAP(H₂) + FH1. Additional quantum corrections to the kinetic energy (QCKE) are used to determine the enthalpy, heat capacities, isochoric pressure coefficient, isobaric thermal expansion coefficient, Joule–Thomson coefficient and the speed of sound. At cryogenic conditions, using QCKE yields close agreement with reference values for these properties.

Molecular simulations are reported for the thermodynamic properties of Ne at temperatures between 30 and 300 K and pressures up to 100 MPa using an intermolecular potential that combines ab initio two-body, three-body, and quantum terms. Quantum corrections to the kinetic energy (QCKE) are also applied to the simulation data. Three-body interactions make important contributions to the pressure–volume–temperature behavior, enthalpy, heat capacity, isothermal compressibility, isochoric pressure coefficient, and isobaric thermal expansion coefficient of Ne. In particular, three-body interactions are required to correctly determine the volume and greatly improve the accuracy of enthalpy, isochoric pressure coefficient, and isobaric thermal expansion coefficient. QCKE also make an important contribution to the properties of Ne that has not been previously recognized. The addition of QCKE means that the caloric thermodynamic properties of Ne can often be determined a priori to an accuracy comparable to that of the reference data.

The isobaric heat capacity (Cp) is frequently used as a benchmark property whenever a new model is proposed or when comparing different force fields with classical molecular dynamics (MD) simulations. However, classical MD is not able to capture the quantum effects inherent in fluids and researchers have opted to apply quantum corrections in the post-processing when evaluating this property. Nevertheless, there is no consensus in the literature regarding the magnitude of quantum corrections for water, with reported values differing by up to a factor of 4. This term can account for up to one third of the Cp value and its erroneous prediction can lead to misleading conclusions. Therefore, we investigate different approaches to properly address quantum corrections when predicting Cp using classical MD. To accomplish this, the quantum correction methods proposed by Horn et al. and Berens et al. are considered, both of which use the single quantum harmonic oscillator approach but employ different strategies to address the frequency space. Two flexible polarizable water models are used in the evaluation, iAMOEBA and AMOEBA14. We show that the method of Berens et al. is a more robust approach to obtain the quantum corrections, as it accounts for all the frequency space by employing the vibrational spectra of the corresponding model under evaluation, making this approach a fully model-based method to determine Cp from MD. In addition, its capability of capturing the shift of low-frequency modes with temperature results in improved performance over the method of Horn et al.

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.

We report molecular simulations using a relatively soft interatomic potential to determine the nature of melting under extreme conditions. At temperatures and pressures above the triple point of normal materials, the density range of two-phase solid and fluid equilibria is bounded by freezing and melting curves. We address the unresolved issue of the termination of these boundaries, i.e., whether the melting curve of a solid terminates in a critical point, exhibits a maximum, goes to an asymptotic limit, or continues indefinitely. Significantly, we observe a negative change in volume upon melting at high pressures, which is normally observed only for water. We provide unequivocal evidence that the densities of the meeting and freezing lines can merge at a melting temperature maximum point. This could be a general feature of "soft" atomic fluids at extreme pressures.