Asymptotic approximation for ionic-covalent configuration mixing in hydrogen and alkali hydrides

Abstract

A pseudo-potential treatment of ionic-covalent configuration interaction or curve-crossing is given which employs simple but asymptotically exact wavefunctions and avoids introducing orbital approximations. The asymptotic wavefunctions are determined solely from known atomic parameters: the ionization potential of the donor species and the electron affinnity and Slater atomic radius of the acceptor. The splitting ΔV(R _c) between the adiabatic potential curves at the diabatic ionic-covalent crossing radius R _c and the potential curves for R⪸R _c are derived from two and three-state secular equations. Results are given for excited electronic states of H₂, particularly the B ¹Σ _u ⁺ state which involves interaction of H(1s)+H(2s, 2p) and H⁺+H^-(1s²) configurations, and for the ground and several excited ¹Σ⁺ states of LiH and other alkali hydrides. Comparisons with spectroscopic data and various theoretical calculations show that for large R _c the simple asymptotic approximation gives good accuracy, within 10–20 per cent for R _c⪸5–6 bohr units. A purely coulombic diabatic potential for the ionic state is also shown to be more realistic than the polarized ion approximation customarily used in previous work. The polarization appears to be largely quenched as a consequence of the extreme diffuseness of the hydride ion.