On the role of coupled-clusters' full triple and perturbative quadruple excitations on rovibrational spectra of van der Waals complexes.

The importance of coupled clusters iterative triple and noniterative quadruple, T (Q) , electron excitations on the potential energy surfaces (PESs) of van der Waals molecules is examined for H 2 –CO. The evaluation of this importance is performed by comparing theoretical spectra with experiment. Somewhat surprisingly, although the T (Q) contributions to interaction energies are below 3%, the inclusion of them reduces the error of theory with respect to experiment by an order of magnitude and qualitative agreement of the spectra in some regions is achieved only when they are included. The main reason for these observations is that the T (Q) effects significantly change the anisotropy of the PES. [ABSTRACT FROM AUTHOR]