Theoretical Chemistry  Accounts 101, 282-291 (1999)

Tatiana Koronaa, Bogumil Jeziorskia, Robert Moszynskia, and Geerd H.F. Diercksenb

a Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
b Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, D-85740 Garching, Germany

Degenerate Symmetry-Adapted Perturbation Theory of Weak Interactions between Closed- and Open-Shell Monomers. Application to Rydberg States of Helium Hydride


Symmetry-adapted perturbation theory is extended to the (quasi)degenerate, open-shell case. The new formalism is tested in the calculations of the interaction energies for a helium atom in the ground state interacting with an excited hydrogen atom. It is shown that the method gives satisfactory results in low orders if the coupling with higher Rydberg states of the dimer is small, as it is the case for the  A2$\Sigma$+, B2$\Pi$, E2$\Pi$, 32$\Pi$, and 12$\Delta$ states of HeH. For the C2$\Sigma$+ state the convergence of the method is very slow, but it can be improved by including the n=3 states into the model space.

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Tatiana Korona 2003-01-16