Convergence behaviour of multireference perturbation theory: An indicator

An approach to investigate the convergence behaviour of a (multireference) Moller-Plesset perturbation series is proposed in which the series is evaluated up to arbitrary order without reference to excitations beyond doubles. It is shown that this approach qualitatively reproduces the convergence behaviour of the complete perturbation series for stretched H-2 molecules and H(2)0 molecules. The MRMP approach does show improved convergence characteristics compared with single reference MP for stretched H-2 molecules. Applications of the approach to O-2 at 4.0 a(0), and N-2 at 5.0 a(0) show that the MRMP series is divergent for these cases. Furthermore, in the latter case it was found that the multireference series shows much stronger divergence than the single reference series.