Extension of random-phase approximation preserving energy weighted sum rules: An application to a 3-level Lipkin model - art. no. 014317

A limitation common to all extensions of random-phase approximation includi ng only particle-hole configurations is that they violate to some extent th e energy weighted sum rules. Considering one such extension, the improved R PA (IRPA), already used to study the electronic properties of metallic clus ters, we show how it can be generalized in order to eliminate this drawback . This is achieved by enlarging the configuration space, including also ele mentary excitations corresponding to the annihilation of a particle (hole) and the creation of another particle (hole) on the correlated ground state. The approach is tested within a solvable 3-level model.