INTERATOMIC POTENTIAL ENERGY HYPERSURFACES FOR MOLECULES

Recent theoretical advances have placed the description of diatomic potential states (ground and excited) by highly accurate non-orthogonal multiconfiguration valence bond (MCVB) wave functions within reach. The methodological contributions made here include the development of techniques for efficiently handling double-zeta and polarization type basis functions for use in the MCVB wave functions. Specifically, an extensive ab initio MCVB calculation for the ground state of hydrogen fluoride is discussed. An efficient method for the modification of atomic orbital basis functions, to describe the process of molecule formation, is reported. Also, various configuration selection schemes for the MCVB wave function are discussed. The most extensive calculation, involving 498 non-orthogonal valence bond structures, gave (experimental values in parenthesis): d(,e) = 5.68 eV (6.18 eV) and r(,e) = 1.76 bohr (1.7325 bohr). These results are the product of the most extensive ab initio MCVB calculations yet performed for hydrogen fluoride and compare quite well with more extensive orthogonal configuration based calculations. The calculations show quite clearly the perturbation of the hydrogen atom by the fluorine atom as the molecule forms. Also, energy curves for the ground state and the first seven excited states of ArH have been calculated. Important features of the excited curves include two distinct avoided crossings between the lowest Ar*H and the highest ArH* potential energy curves. Using the curves we have made a theoretical analysis of quenching of metastable Ar by collision with H at room temperature. An estimate of the curve switching probability is given by the Landau-Zener formula and leads to a theoretical value of the quenching rate approximately ten times the experimental. In light of the many approximations involved this qualitative agreement is satisfactory and provides a rationale to explain the anomalously high rate constant for the quenching reaction. Structural features of the interacting potential curves are discussed in terms of the diabatic states involved