ON THE INHERENT DIVERGENCE IN THE MOLLER-PLESSET SERIES - THE NEON ATOM - A TEST-CASE

It has recently been shown that Moller-Plesset perturbation calculatio ns may diverge for single configuration dominated systems in extended basis sets. We examine the origin of this divergence using the neon at om as a test example. The lowest energies for total symmetric states a re calculated for values of the perturbation parameter strength z is a n element of[-1, 1]. For z approximate to(-0.9)-(-0.8) avoided crossin gs between the Hartree-Fock configuration dominated state and states d ominated by quintuple and higher excitations are observed. The origin of the divergence is therefore ''back door'' intruder states, that are highly excited relative to the Hartree-Fock state.