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The variation in the singlet-triplet energy gap of diphenylcarbene (DPC) upon interaction with hydrogen (water and methanol) or halogen bond (XCF 3 , X = Cl, Br, I) donors to form van der Waals (vdW) complexes is investigated in relation to the electrostatic and dispersion components of such intermolecular interactions. The domain-based local pair natural orbital coupled cluster method, DLPNO-CCSD(T), is used for calculating accurate single-triplet energy gaps and interaction energies for both spin states. The local energy decomposition scheme is used to provide an accurate quantification to the various interaction energy components at the DLPNO-CCSD(T) level. It is shown that the formation of vdW adducts stabilizes the singlet state of DPC, and in the case of water, methanol, and ICF 3 , it reverses the ground state from triplet to singlet. Electrostatic interactions are significant in both spin states, but preferentially stabilize the singlet state. For methanol and ClCF 3 , London dispersion forces have the opposite effect, stabilizing preferentially the triplet state. The quantification of the energetic components of the interactions through the local energy decomposition analysis correlates well with experimental findings and provides the basis for more elaborate treatments of microsolvation in carbenes.

A test set of 12 aryl carbenes (AC12) is compiled with the purpose of establishing their adiabatic singlet-triplet energy splittings using correlated wave function based methods. The set covers both singlet and triplet ground state aryl carbenes, as well as a range of magnitudes for the ground state to excited state gap. The performance of coupled cluster methods is examined with respect to the reference wave function, the basis set, and a number of additional methodological parameters that enter the calculation. Inclusion of perturbative triples and basis set extrapolation with a combination of triple and quadruple-ζ basis sets are both required to ensure high accuracy. When canonical coupled cluster calculations become too expensive, the domain-based local pair natural orbital approach DLPNO-CCSD(T) can be used as a reliable method for larger systems, as it achieves a mean absolute error of only 0.2 kcal/mol for the singlet-triplet gaps in the present test set. Other first-principles wave function methods and selected density functional methods are also evaluated. Second-order Møller-Plesset perturbation theory approaches are only applicable in conjunction with orbital optimization (OO-MP2). Among the representative density functional methods tested, only double hybrid functionals perform sufficiently accurately to be considered useful for systems with small singlet-triplet gaps. On the basis of the reference coupled cluster results, projected gas-phase free energies are reported for all aryl carbenes. Finally, the treatment of singlet-triplet gaps in solution is discussed in terms of both implicit and explicit solvation.