Multireference many-body perturbation theory calculations on negative ions with small electron detachment energies

We present a simple method of ab initio calculations on the systems composed of a closed-shell-like molecule (atom) and a weakly bound electron via constructing a state-specific effective Hamiltonian in a model space generated by placing the extra electron on virtual self-consistent field orbitals of the neutral species in all the possible ways. Correlation contributions are incorporated into the effective Hamiltonian by the second-order multipartitioning many-body perturbation theory. The method prevents the appearance of intruder states without destroying the exact size consistency of results and ensures a proper description of the dynamic correlation effects on the spatial localization of the extra electron and continuous behavior of computed characteristics under variations of molecular geometry. A detailed study of perturbative solutions for a model problem with parameter-dependent Hamiltonian and test calculations of Ca, Sr, and Ba atomic anions are reported. The method is also applied to the description of the hypothetical symmetrical isomer of water dimer anion with the extra electron trapped between the water molecules. (C) 2003 Wiley Periodicals, Inc.