Assessment of the Effects of Anisotropic Interactions among Hydrogen Molecules and Their Isotopologues: A Diffusion Monte Carlo Investigation of Gas Phase and Adsorbed Clusters

Abstract

Despite the fact that the para-hydrogen molecule (p-H) and its isotopomers (o-D and p-T) are commonly modeled as spherical objects due to the large separation between rotational states, there may be situations (e.g. adsorption in pores and on surfaces) in which such an approximation neglects important degrees of freedom (i.e. the rotational ones) and introduces uncontrolled biases in the predicted properties. To better understand when approximating such molecules as spheres introduces shortcomings in their representation, we employed diffusion Monte Carlo to simulate small/medium-sized molecular aggregates, either isolated in space or experiencing external model potentials, to compute energetic quantities and distribution functions. These were chosen to mimic situations possibly occurring in real systems, in which orientational isotropy is broken. The comparison between isolated clusters with molecules described as rigid rotors with a 4D potential or as spheres interacting via Adiabatic Hindered Rotor models shows that neither energetic nor structural quantities are affected by reducing the systems dimensionality. The orientational degrees of freedom of the rotors remains largely uncoupled from translational ones whatever the molecular mass. The same happens for rotors interacting with a frozen hydrogen molecule in the vicinity of a repulsive surface. Deviating from such behavior are molecular aggregates interacting with potentials mimicking the presence of ionic adsorption sites inside porous materials. Such difference is ascribable to the markedly anisotropic and longer ranged nature of those interactions, both features being relevant in defining the adsorption energy of the molecular species.

Snowball formation for Cs+ solvation in molecular hydrogen and deuterium

“…Our goal is to investigate whether well-defined and compact shells are formed and what their structure is. In addition, since the H 2 -Cs + interaction is very anisotropic, we believe that it is worth studying the H 2 /D 2 orientational effects 10,31,32 by explicitly taking into account their rotational degrees of freedom and comparing with the more widely used pseudoatom model. The importance of three-body (3B) induction forces [33][34][35][36] is assessed as well.…”

Electrolyte clusters as hydrogen sponges: diffusion Monte Carlo simulations

“…Nevertheless, these results are in stark contrast with those uncovered in previous investigations of hydrogen aggregated to neutral ammonia molecules. 33–35,40,47 The lithium cation is quite small compared to the solvent dipoles, and clearly it size and charge are responsible for some of the differences we find in this study, albeit, we have sufficient evidence to expect that a neutral cluster of THF-like Stockmayer particles may cage hydrogen as well. In fact, we observe that the fifth hydrogen molecule is between solvation layers of EC106 rather than on the surface.…”

“…Moreover, computer simulations of hydrogen storage with any type of substrate can be very insightful only if quantum simulation tools such as Diffusion Monte Carlo, [29][30][31][32] are used to handle significant nuclear quantum effects produced by the relatively small mass of the hydrogen molecule. [33][34][35] Much of our recent development of theoretical and computational tools has been focused on improving quantum simulations of structured particles such as point dipoles in the Stockmayer model 36 and the rigid hydrogen. 11,34,35,[37][38][39][40][41][42][43][44][45][46][47][48] Therefore, we feel compelled to begin this journey of exploration in earnest.…”

“…[33][34][35] Much of our recent development of theoretical and computational tools has been focused on improving quantum simulations of structured particles such as point dipoles in the Stockmayer model 36 and the rigid hydrogen. 11,34,35,[37][38][39][40][41][42][43][44][45][46][47][48] Therefore, we feel compelled to begin this journey of exploration in earnest.…”