On the inherent divergence in the Møller-Plesset series. The neon atom - A test case

It has recently been shown that Møller-Plesset perturbation calculations may diverge for single configuration dominated systems in extended basis sets. We examine the origin of this divergence using the neon atom as a test example. The lowest energies for total symmetric states are calculated for values of the perturbation parameter strength z ∈[-1, 1], For z ≈ ( -0.9)-( - 0.8) avoided crossings between the Hartree-Fock configuration dominated state and states dominated 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.