XVIth International Workshop on
Quantum Systems in
Chemistry and Physics
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Koopmans' theorem in the ROHF method. General formulation
Boris N. Plakhutin
Laboratory of Quantum Chemistry, G.K.Boreskov Institute of Catalysis, Russian Academy of Sciences
Novosibirsk 630090, Russia
A general formulation of Koopmans’ theorem (KT) is derived in the ROHF method for molecular and atomic systems with arbitrary orbital occupancies and total electronic spin. The key point of the formulation is the new variational condition introduced in the ROHF method in addition to the familiar variational principle.

The new condition ensures the stationarity of the ionized energy (i.e., of the ionization potential or electron affinity) defined in Koopmans’ approximation with respect to a variation of orbitals within the ionized electronic shell. Taken together, the variational principle and the new condition define the special ( canonical) form of the ROHF Hamiltonian whose eigenvalues (orbital energies) satisfy KT for the whole energy spectrum.

The talk will focus on the deep relation between the variational conditions underlying the present (canonical) ROHF method and the limited configuration interaction (CI) approach. The practical applicability of the theory is verified by comparing the KT estimates [4] of the vertical ionization potentials I2s and I2p and electron affinities A2p for the second row open-shell atoms (Li to F) with the respective experimental data. This study was supported by the Russian Fund for Basic Research (grant 09-03-00113) and the Russian Academy of Sciences (grant OXHM/5.1.9).

1. B.N. Plakhutin, E.V. Gorelik, and N.N. Breslavskaya, J. Chem. Phys. 125, 204110 (2006).
2. B.N. Plakhutin and E.R. Davidson, J. Phys. Chem. A. 113, 12386 (2009).
3. E.R. Davidson and B.N. Plakhutin, J. Chem. Phys. 132, 184110 (2010).
4. B.N. Plakhutin, submitted.


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