Reaching High Accuracy for Energetic Properties at Second-Order Perturbation Cost by Merging Self-Consistency and Spin-Opposite Scaling

Quantum chemical methods dealing with challenging systems
while retaining low computational costs have attracted attention.
In particular, many efforts have been devoted to developing new methods
based on second-order perturbation that may be the simplest correlated
method beyond Hartree–Fock. We have recently developed a self-consistent
perturbation theory named one-body Møller–Plesset second-order
perturbation theory (OBMP2) and shown that it can resolve issues caused
by the noniterative nature of standard perturbation theory. In this
work, we extend the method by introducing spin-opposite scaling to
the double-excitation amplitudes, resulting in the O2BMP2 method.
We assess the O2BMP2 performance on the triple-bond N2 dissociation,
singlet–triplet gaps, and ionization potentials. O2BMP2 performs
much better than standard MP2 and reaches the accuracy of coupled-cluster
methods in all cases considered in this work.