The benzene–argon complex: A ground and excited state ab initio study

Equilibrium dissociation energies $D_e$ of the benzene–argon van der Waals complex are calculated in the ground state $S_0$ and in the excited state $S_1$ using integral-direct coupled cluster methods. The results confirm previous investigations of $S_0,$ showing that high quality correlation consistent basis sets and connected triple excitations are imperative for a good description of the van der Waals complex. We estimate the CCSD(T) dissociation energy $D_e\text{=389±2\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ for the ground state $S_0.$ Using the CCSD linear response approach the frequency shift (redshift) ${\mathrm{\delta \nu }}_e\text{=19\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ is obtained. Accurate spectroscopic structural data and frequency shifts ${\mathrm{\delta \nu }}_0$ for the $6_0^1$ band of the $S_1{\mathrm{\leftarrow S}}_0$ transition are available for most of the benzene–rare gas atom complexes. However, the experimental determination of absolute dissociation energies of these complexes is connected with much larger uncertainties. The theoretical result agrees very well with the experimentally available redshift, showing that integral-direct coupled cluster methods will become an important tool in the study of van der Waals complexes in the future.