An approximation to the polarization propagator using an optimized antisymmetrized geminal power (AGP) wave function as a reference state is discussed. As an application, potential energy curves and spectroscopic constants for some of the low-lying electronic states in Li2 are given. Transition moments and radiative lifetimes for the experimentally observed B 1Πu–X 1Σ+g and A 1Σ+u–X 1Σ+g transitions are also presented. Excellent agreement with experiment and theoretical calculations of multiconfigurational self-consistent field (MCSCF) type is obtained. off

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The theory for polarization propagator calculations based on a correlated reference state in the form of a spin singlet restricted antisymmetrized geminal power (AGP) is discussed. Particular attention is paid to the calculation of excitation energies and transition moments for the molecules over all internuclear separations, i.e., not just single point vertical energies, which is done in standard RPA or in higher-order treatments using perturbation methods. Simple applications to the Be atom, and the LiH and Li2 molecules are used to illustrate how this simple correlated state, which is completely determined by the choice of orbital basis (i.e., no choice of configurations is necessary), can cure the notorious consistency problems and the instabilities against the breaking of symmetry, which plague standard RPA based on a Hartree–Fock (HF) single determinantal reference state.

A discussion is given on some of the properties of the first-and second-order reduced density matrices and their occupation numbers for antisymmetrized geminal power (AGP) wavefunctions.Improvements on this type of functions are discussed in terms of a projected AGP.