Mixing of ionic and covalent configurations for NaH, KH, and MgH+. Potential energy curves and couplings between molecular states

Robert W. Numrich, Donald G. Truhlar

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Configuration mixing between ionic and covalent configurations in the hydrides NaH, KH, and MgH+ has been studied using a two-electron valence bond model in which the valence electrons are assumed to interact with the metal core through a Hellmann-type pseudopotential. Adiabatic potential energy curves are reported for the X1+, A1+, B1+, a3+, b3+, c3+, 1Π and 3Π states arising from the ns, np, and (n + 1)s atomic states of the metal atoms. For each hydride a polarization potential was added to the diabatic ionic potential curve and this potential was adjusted in such a way that the binding energy of the A1+ adiabatic state is in good agreement with the experimental value. The A1+ potential energy curves are also in good agreement with RKR curves determined from experiment although they are shifted slightly to larger values of the internuclear distance. For each diatomic system, coupling matrix elements between electronic states were calculated in a nonorthogonal diabatic representation, in a symmetrically orthogonalized diabatic representation, and in the adiabatic representation. The elements of the nonorthogonal diabatic Hamiltonian matrix and the overlap matrix were used to calculate the Landau-Zener-Stueckelberg parameter AW which is the separation between adiabatic potential energy curves at the position of an avoided crossing in a two-state approximation. Comparison with values of AW obtained earlier by Bates and Boyd and by Grice and Herschbach shows fair agreement between the values obtained here and those obtained by Bates and Boyd while the values obtained by Grice and Herschbach are two to three times larger. The elements of the nonorthogonal diabatic Hamiltonian matrix and the elements of the inverse square root of the overlap matrix were fit to analytic forms as functions of the intemuclear distance. These forms were used to evaluate the part of the coupling matrices between adiabatic states which arises from the R dependence of the transformation matrices between the three electronic representations. The results show that coupling between adiabatic states in these molecules is important over intervals as large as 10 bohrs and serious questions are raised concerning the Landau-Zener-Stueckelberg model for treating the problem of avoided crossings in scattering calculations involving these systems.

Original languageEnglish (US)
Pages (from-to)2745-2766
Number of pages22
JournalJournal of physical chemistry
Issue number25
StatePublished - 1975

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