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

Despite the fact that the para-hydrogen molecule (p-H2) and its isotopomers (o-D2 and p-T2) 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