Knowledge Bank

The Fock space Hamiltonian H - which has eigenstates of arbitrary number of electrons in a given nuclear framework - is transformed to an effective Hamiltonian L, which has (in principle) the exact eigenvalues but simple model states as $eigenfunctions.1$ The method can be regarded as a non-perturbative generalization of quasidegenerate many-body perturbation theory. It is formulated in spinfree form in the framework of the unitary group. In contrast to the MCSCF-CI method dynamical correlation is treated first and near-degeneracy effects at the end. All interesting states are obtained from a small CI based on L. Various states of a neutral molecule and its ions are obtained from the same L. In the case of avoided crossings one can choose to get either adiabatic or diabatic states. Results for spectral transition energies and ionization potentials of atoms and small molecules are encourageing.