The benzene–argon complex: A ground and excited state <i>ab initio</i> study

Equilibrium dissociation energies De of the benzene–argon van der Waals complex are calculated in the ground state S0 and in the excited state S1 using integral-direct coupled cluster methods. The results confirm previous investigations of S0, 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 De=389±2 cm−1 for the ground state S0. Using the CCSD linear response approach the frequency shift (redshift) δνe=19 cm−1 is obtained. Accurate spectroscopic structural data and frequency shifts δν0 for the 601 band of the S1←S0 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.