Iterative perturbation calculations of excited ground and excited state energies from multiconfigurational zerothorder wavefunctions
Abstract
A method is proposed to calculate the effect of configuration interaction by a RayleighSchrödinger perturbation expansion when starting from a multiconfigurational wavefunction. It is shown that a careless choice of H_{0} may lead to absurd transition energies between two states, at the first orders of the perturbation, even when the perturbation converges for both states. A barycentric defintion of H_{0} is proposed, which ensures the cancellation of common diagrams in the calculated transition energies. A practical iterative procedure is defined which allows a progressive improvement of the unperturbed wavefunction ψ^{0}; the CI matrix restricted to a subspace S of strongly interacting determinants is diagonalized. The desired eigenvector ψ^{0} of this matrix is perturbed by the determinants which do not belong to S. The most important determinants in ψ^{1} are added to S, etc. The energy thus obtained after the secondorder correction is compared with the ordinary perturbation series where ψ^{0} is a single determinant. For the ground state, this procedure includes, besides the whole secondorder correction, the most important terms of the third and fourth orders. The question of orthogonality of excited states is discussed. This technique, hereafter called CIPSI, has been tested on the ground and several excited states of H_{2}, Ne, and MgO, showing both a rapid convergence of the calculated transition energy and the importance of correlation effects on transition energy.
 Publication:

Journal of Chemical Physics
 Pub Date:
 June 1973
 DOI:
 10.1063/1.1679199
 Bibcode:
 1973JChPh..58.5745H