Constant denominator perturbative schemes and the partitioning technique

摘要

With the aid of Lowdin's partitioning theory,an infinite series for the eigenvalue of the Schrodinger equation is derived which does not ocntain energy differences in denominators.The resulting formulae are compared to those of constant denominator methods,such as perturbation theory within the Unsold approxiation and the connected moment expansion (CMX).Teh Unsold formulae are eawily obtained from pratitioning theory by a suitable choice of the zero order Hamiltonian.Optimizing the value of the energy denominator using the first order wave function in a size-consistent way,the third order Unsold correction vanishes,and the corresponding energy correction formula of the CMX is recovered atthe second order.