Performance of density functional theory and orbital-optimised second-order perturbation theory methods for geometries and singlet–triplet state splittings of aryl-carbenes
Reza Ghafarian Shirazi, Dimitrios A. Pantazis, Frank Neese
Abstract
Carbenes are challenging molecular species for quantum chemistry because of the energetic proximity of their singlet and triplet spin states and the sensitive dependence of spin-state energetics on the geometry of the carbene site. Here we use an extended set of aryl-carbenes to evaluate the performance of density functional theory (DFT) approximations as well as of wave function based perturbation theory approaches (orbital-optimised perturbation theory methods OO-MP2 and OO-SCS-MP2) against reference coupled cluster calculations with singles, doubles and perturbative triples conducted with the aid of the domain-based local pair natural orbitals approach, DLPNO-CCSD(T). In addition to the expected functional dependence, our results document a remarkable discordance in the performance of DFT methods in the sense that the functionals that yield the best geometries do not coincide with those that provide the best spin-state energetics. Analysis of the results allows us to propose a series of methods that are expected to perform reliably within certain confidence limits for the title systems. Additionally, methodological issues regarding the reference singlet–triplet gaps obtained by the DLPNO-CCSD(T) approach are discussed.