Single-Root Multireference Brillouin-Wigner Perturbative Approach to Excitation Energies

The state-specific Brillouin-Wigner multireference perturbation theory [which employs Jeziorski-Monkhorst parametrization of the wave function] using improved virtual orbitals, denoted as IVO-BWMRPT, is applied to calculate excitation energies (EEs) for methylene, ethylene, trimethylenemethane, and benzyne systems exhibiting various degrees of diradical character. In IVO-BWMRPT, all of the parameters appearing in the wave function ansatz are optimized for a specific electronic state. For these systems, the IVO-BWMRPT method provides EEs that are in close agreement with the benchmark results and experiments, where available, indicating that the method does not introduce imbalance in the target-specific treatment of closed- and open-shell states involved. The good performance of the present methodology is primarily related to structural compactness of the formalism. Overall, present findings are encouraging for both further development of the approach and chemical applications on the energy differences of strongly correlated systems.

Automated summary

The state-specific Brillouin-Wigner multireference perturbation theory employing Jeziorski-Monkhorst parametrization with improved virtual orbitals, known as IVO-BWMRPT, is applied to calculate excitation energies for various systems with diradical character. The method provides results close to benchmark and experimental results, indicating balance in treatment of closed- and open-shell states. The good performance is attributed to the structural compactness of the formalism, showing promise for further development and applications in energy differences of strongly correlated systems.