Convergence properties of multireference many-body perturbation theory

The applicability of multireference many-body perturbation theory is considered by numerically investigating the practical convergence properties of a common variant. Perturbation energies through 20th order are reported for BeH₂ at geometries near the Be{→}H₂ symmetric-insertion transition state and for BH in its equilibrium region. The recursive perturbation-theory equations are solved using a computationally intensive, but conceptually simple, configuration-based algorithm. In the study of diverse regions of molecular potential-energy surfaces, the difficulties involved in selecting appropriate zeroth-order models, which consist of the choice of reference functions, orbitals, and zeroth-order Hamiltonian, are addressed. We also consider convergence-acceleration techniques such as series resummation using Pad\'e approximants.