ABINITIO STUDY OF THE POTENTIAL-ENERGY SURFACE FOR THE INTERACTION OF NA+ WITH H2 AND THE GEOMETRIES AND ENERGIES OF NA+(H2)N, N = 2-4

A near-Hartree-Fock-limit calculation of the dependence of the interaction between Na+ and H-2 on distance, orientation, and H-2 bond length has been carried out at 291 geometries and fit to an analytic, nonpiecewise potential function with correct long-range behavior. The H atom basis set was chosen to give accurate polarizability components and quadrupole moment for H-2, and each point on the surface was corrected for a small basis-set superposition error using the counterpoise method. With the large basis sets employed, electron correlation effects are shown to be nearly negligible, changing the well depth at the minimum point on the surface by less than 1%. Equilibrium geometry, energy, and vibrational frequencies are compared with recent ab initio studies, and the repulsion at short range with experimental estimates derived from scattering studies. In the cluster calculations, the additional H-2's are arranged in the classic electron-pair-repulsion geometries with the H-H bond axes perpendicular to the Na+-H-2 axes; a less constrained optimization for n = 2 shows this to be realistic. In the complex, the H-2's are too distant to repel each other by overlap even for n = 4; the determining factor appears to be repulsion between H2 dipoles induced by Na+. Due to this repulsion, the (classical) energy released when an H-2 is added declines slightly as n grows. The structures are nonrigid with respect to translational or rotational motion of H-2, SO long as its internal bond axis remains within the solvation shell.