Second-order state-specific multireference Møller-Plesset perturbation theory (SS-MRMPPT) applied to geometry optimization.

The performance of a numerically oriented gradient scheme for the previously introduced second-order state-specific multireference Møller-Plesset perturbation theory (SS-MRMPPT) has been tested to compute certain geometrical parameters (such as bond lengths and angles). Various examples [H2O, O3, N2H2, C2H4, C2H2F2, 1,3-butadiene (C4H6), cyclobutadiene (C4H4), and 2,6-pyridynium cation (C5NH4(+))] have been presented to validate the implementation of the SS-MRMPPT gradient approach. To illustrate the reliability of our findings, comparisons have been made with the previously reported theoretical results. The accuracy of our calculations has further been assessed by comparing with the experimental results whenever available. On the basis of the present work, we arrive at the conclusion that the SS-MRMPPT gradient scheme has substantial potential in computing the geometrical parameters for several rather diverse molecular systems, whether charged or neutral and having the closed-shell ground state or being open-shell radicals or biradicals or strongly perturbed by intruders. It is worthwhile to emphasize that the present work represents the first systematic application of the SS-MRMPPT numerical gradient approach.