Physical nature of silaneMIDLINE HORIZONTAL ELLIPSIScarbene dimers revealed by state-of-the-art ab initio calculations

Using the SAPT2 + 3(CCD)delta MP2 method in complete basis set (CBS) limit, it is shown that the interactions in the recently studied silaneMIDLINE HORIZONTAL ELLIPSIScarbene dimers are mainly dispersive in nature. Consequently, slow convergence of dispersion energy also forces slow convergence of the interaction energy. Therefore, obtaining very accurate values requires extrapolation of the correlation part to the CBS limit. The most accurate values obtained at the CCSD(T)/CBS level of theory show that the studied silaneMIDLINE HORIZONTAL ELLIPSIScarbene dimers are rather weakly bound, with interaction energies ranging from about -1.9 to -1.3 kcal/mol. Comparing to CCSD(T)/CBS, it will be shown that SCS-MP2 and MP2C methods clearly underestimate and methods based on SAPT2+ and having some third-order corrections, as well as the MP2 method, overestimate values of interaction energies. Popular SAPT(DFT) method performs better than SCS-MP2 and MP2C; nevertheless, underestimation is still considerable. The underestimation is slightly quenched if third-order dispersion energy and its exchange counterpart is added to the SAPT(DFT). The closest value of CCSD(T)/CBS has been given by the SAPT2 + (3)(CCD)delta MP2 method in quadruple-zeta basis set. (c) 2019 Wiley Periodicals, Inc.