Ab initio studies of π•••π interactions:: The effects of quadruple excitations

To examine the effects of including quadruple excitations in the electron correlation treatment when computing the binding energy of van der Waals dimers, we have calculated MP2, CCSD, CCSD(T), and CCSD(TQ) interaction energies for several van der Waals complexes ranging from helium dimer to furan dimer. Through comparison of CCSD(TQ) and CCSD(T) binding energies the first direct assessment of the effects of quadruple excitations on pi(...)pi interactions is presented. The influence of triple excitations is assessed not only in the conventional manner that permeates the literature (comparison of CCSD(T) and MP2 interaction energies) but also via comparison of CCSD(T) and CCSD data. In systems exhibiting significant electron delocalization, it is well-known that triples contributions to binding energy are quite large (on the order of 1 kcal mol(-1)). In these cases, quadruple excitations are nonnegligible and tend to be an order of magnitude smaller than the correction to the binding energy from triple excitations (on the order of 0.1 kcal mol(-1)). The largest quadruples correction observed was +0.2 kcal mol(-1) for the furan dimer.