Rayleigh-Schrodinger perturbation theory approximation

Whereas the multi-reference Rayleigh-Schrodinger perturbation theory approximates a manifold of states simultaneously, the multi-reference Brillouin-Wigner perturbation theory approach is applied to a single state - it is said to be state-specific . The multi-reference Brillouin-Wigner perturbation theory avoids the intruder state problem. If a particular Brillouin-Wigner-based formulation is not a valid many-body method, then a posteriori correction can be applied. This correction is designed to restore the extensivity of the method. This extensivity may be restored approximately... [Pg.31]

The method is based on the following procedure.33 All possible doubly excited configurations are generated from the Hartree-Fock function and their contributions to the second-order Rayleigh-Schrodinger perturbation theory energy computed. Approximately 100 of the most important are used for a Cl calculation, all singly... [Pg.82]

Even-order Invariants for Rayleigh-Schrodinger Perturbation Theory. Full fourth-order correlation energy calculations are now possible and it is, therefore, important to have fourth-order forms which share the invariance properties of the [2/1] Pade approximants in third-order. The expression... [Pg.45]

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