Computational study of molecular hydrogen in zeolite Na-A. I. Potential energy surfaces and thermodynamic separation factors for ortho and para hydrogen

We simulate H-2 adsorbed within zeolite Na-A. We use a block Lanczos proced ure to generate the first several (9) rotational eigenstates of the molecul e, which is modeled as a rigid, quantum rotor with an anisotropic polarizab ility and quadrupole moment. The rotor interacts with Na cations and O anio ns; interaction parameters are chosen semiempirically and the truncation of electrostatic fields is handled with a switching function. A Monte Carlo p roceedure is used to sample a set of states based on the canonical distribu tion. Potential energy surfaces, favorable adsorbtion sites, and distributi ons of barriers to rotation are analyzed. Separation factors for ortho-para hydrogen are calculated; at low temperatures, these are controlled by the e ase of rotational tunneling through barriers. (C) 1999 American Institute o f Physics.