The (H2)2 potential surface and the interaction between hydrogen molecules at low temperatures

Abstract

The (H2)2 rigid rotor interaction potential has been calculated for the intermolecular distance range R = 3.0–11.0 a.u. for six relative orientations to estimate both the isotropic and anisotropic components of the full intermolecular potential. A partially optimized basis set limited in size to 78 independent Gaussian functions was used throughout the energy calculations, which required only very small corrections for basis set unsaturation effects. Correlation effects were computed both at the variational (single and) double excitation PNOCI level and using the CEPA2-PNO approximation to estimate higher order excitation effects. While the latter rigid-rotor surface may overestimate the strength of the H2–H2 interaction in the vicinity of the well by a few wave numbers in the rigid-rotor PNOCI surface from the present study, which we regard as an upper bound to the true rigid-rotor surface, is also slightly deeper than almost all previous theoretical and empirical ’’fit’’ potentials in the well region. Since low energy scattering experiments lack sensitivity to the precise well depth because of the long de Broglie wavelengths appropriate to scattering at energies comparable with the well depth, the significance of the current disagreement on the well depth between the rigid-rotor level of the theory (which puts it at least 27.5 cm−1) and experiment is uncertain. However, coupled with the present uncertainty stemming from the CI formalism of ∼±3 cm−1, is a further uncertainty due to the expected increase in intramolecular zero-point energies due to hindered H2 vibration within the dimer. A zero-point increase within each H2 at Re of 0.1% would effectively raise the rigid-rotor potentials by some 4 cm−1, bringing the well depth into line with experimental estimates. In the low energy part of the repulsive wall of the potential (where the relative importance of these small effects diminishes), which has been probed rather sensitively by recent rotationally inelastic scattering experiments and which strongly influences solid H2 properties, we find that both the present PNOCI and CEPA2 surfaces lie in between the respective semiempirical fit surfaces for the H2–H2 ’’pair’’ interaction. At the zero crossing of the isotropic component of the potential, we find excellent agreement between the present theoretical estimates of R0(2.99±0.03 Å) and the most recent experimental evaluation of the parameter (2.98±0.03 Å) by Buck et al. [J. Chem. Phys. 74, 535 (1981)].

An interatomic potential for saturated hydrocarbons based on the modified embedded-atom method

“…All atoms were constrained at each distance increment, and the energy was calculated by subtracting the total energy of the structure at infinite atomic distance from the actual energy at each distance increment. The FP data for the hydrogen dimer were taken from the work of Burton et al 46 , where four different molecular configurations, denoted as collinear coplanar (T), linear (L), parallel or rectangular (P), and crossed or elongated tetrahedron (C), were considered. The configurations are depicted in Fig.…”

“…First-principles (FP)46 versus MEAM-calculated interaction energy curves for (H 2 ) 2 (hydrogen dimer). The molecular configurations are a) collinear coplanar (T), b) linear (L), c) parallel or rectangular (P), and d) crossed (C) as reported in the work of Burton et al46 The atoms are constrained during energy calculation at each distance increment. The double arrow indicates the coordinate that is being varied.arXiv: 1305.2759 physics.chem-ph Nouranian et al (2014), 1-29 | 17 First-principles (FP) 47 versus MEAM-calculated interaction energy curves for (CH 4 ) 2 (methane dimer).…”

Theory of the reentrant quantum rotational phase transition in high-pressure HD

“…In summary, in the range of pressure of interest, 0-100 GPa, the EQQ interaction is the leading contribution. The associated coefficients B l 1 l 2 l (R 12 ) have been extensively studied, for instance, by Schaefer et al 35 and by Burton et al 36 (see also Refs. [37][38][39].…”

Spherical harmonics representation of the potential energy surface for the H2⋯H2 van der Waals complex

“…The nature of the intermolecular interactions of hydrogen molecules [1][2][3][4][5][6][7][8][9] has been the subject of works for years. Previously, we focused on the development of the potential energy surface (PES) or performed accurate calculations for the interaction energy using the supermolecular approach [10,11].…”