Hartree-Fock solutions as a quasidiabatic basis for nonorthogonal configuration interaction

Using the method of self-consistent field metadynamics, we locate some of the low-energy solutions to the Hartree-Fock (HF) equations on LiF and O-3. The located solutions qualitatively resemble the adiabatic electronic states in these systems. We formulate the method of nonorthogonal Configuration Interaction (CI) to interact these solutions with cubic scaling with system size and quadratic scaling with the number of solutions. The resultant solutions display the avoided crossings and, in O-3, a conical intersection expected of the adiabatic states. In LiF the relevant solutions coalesce and disappear from Unrestricted HF space indicating a more general HF theory is required. (C) 2009 American Institute of Physics. [doi:10.1063/1.3236841]