  We test a set of multiconfigurational wavefunction approaches for calculating
the ground state electron population for a two-site Anderson model representing
a molecule on a metal surface. In particular, we compare (i) a Hartree Fock
like wavefunction where frontier orbitals are allowed to be nonorthogonal
versus (ii) a fully non-orthogonal configuration interaction wavefunction based
on constrained Hartree-Fock states. We test both the strong and weak
metal-molecule hybridization ($\Gamma$) limits as well as the strong and weak
electron-electron repulsion (U) limits. We obtain accurate results as compared
with exact numerical renormalization group (NRG) theory, recovering charge
transfer states where appropriate. The current framework should open a path to
run molecular non-adiabatic dynamics on metal surfaces.
