Ab initio excitation spectrum of the weak H•••CO interaction

The adiabatic interaction energy (IE) in the van der Waals region of the ground 1 Upσ{ + } and excited H{2}S CO a3 electronic states of the H CO complex is studied in the framework of the supermolecule approach at the RHF-CCSD(T) level of theory. Calculations predict a minimum with β = 72°, R = 6.89a and D = 34.10 cm for the ground XA'state. For the excited A' state the minimum occurs at β = 104° and R = 5.90a with D = 75.42 cm. The resulting IE of the excited A'' state reveals two minima separated by a saddle point. The most stable configuration occurs at β = 132°, R = 6.71a and D = 40.03 cm. The corresponding vertical excitation energies and corresponding shifts with respect to the isolated CO molecule are calculated as a guideline for future theoretical and experimental work. In order to investigate the use of less demanding correlation methods, test density functional theory calculations using the mPW1PW exchange-correlation functional are also presented for comparison. © 2008 Springer-Verlag.