The Moeller-Plesset perturbation expansion is a popular technique of quantum chemistry, but recent studies [1] have suggested that this method can yield unreliable results due to poor convergence. Quadratic Pade summation is capable of significantly improving the convergence rate [2,3]. This is because the convergence problems are caused by square-root branch points in the space of the expansion parameter, which are explicitly modeled by these summation approximants. Current work is directed toward further improving the convergence by using repartitioning methods [3] to lessen the effect of the branch points, and toward using quadratic approximants to calculate complex energy eigenvalues corresponding to autoionization resonances. Another focus of current work is a recently developed summation approximant for coupled-cluster theory [4], which shows significant promise for practical applications.

[1] O. Christiansen, et al., Chem. Phys. Lett. 261, 369 (1996); M. L. Leininger et al., J. Chem. Phys. 112, 9213 (2000); and references therein.

[2] D. Z. Goodson, " Convergent Summation of Moeller-Plesset Perturbation Theory," J. Chem. Phys. 112, 4901 (2000).

[3] D. Z. Goodson, " A Summation Procedure that Improves the Accuracy of Fourth-Order Moeller-Plesset Perturbation Theory," J. Chem. Phys. 113, 6461-6464 (2000).

[4] D. Z. Goodson, "Extrapolating the Coupled-Cluster Sequence Toward the Full Configuration-Interaction Limit," Journal of Chemical Physics, in press. [ abstract]