Correlation balance for describing carbenes: An NOF study

Carbenes are a remarkable type of molecules because of the two electrons that might be paired or unpaired, providing a singlet or a triplet state, respectively. The preference for one of these states can be measured by the singlet-triplet energy gap; however, theoretical prediction is challenging when static correlation arises, and an adequate balance with dynamic correlation is required to achieve correct predictions. Piris Natural Orbital Functionals (PNOFs) have been used before to deal with other static correlation problems, but they have suffered from lack of dynamic correlation, which has been solved by coupling PNOF7 with many-body perturbation theory. Recently, the development of a Global Natural Orbital Functional (GNOF) has been proposed with the aim of including dynamic correlation without the need to couple with perturbation theory, thus becoming a promising alternative to study challenging chemical problems. In this work, we applied the PNOF family of functionals to compute the adiabatic singlet-triplet energy gaps of a set of simple carbenes and compared them with those computed with coupled-cluster methods and experimental values when available. We have found that the GNOF achieves promising results due to an intrinsic balance of static and dynamic correlation. In this regard, the GNOF presents errors that are lower than those of PNOF7, comparable to those of NOF-c-MP2, and is capable of predicting the tendency of substituent effects, proving to be suitable for further application to predict general singlet-triplet energy gaps.