Test study on the excitation spectrum of the CO•••Ar van der Waals molecule

In the present contribution, we performed ab initio Moller-Plesset perturbation theory second-order (MP2) calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals CO-Ar molecule in its "near T-shaped" most stable ground state structure, as a guideline for future theoretical and experimental work. These test calculations indicate a ground CO(X(1)Sigma)-Ar(S-1) interaction with R-e = 3.72 Angstrom, D-e = 109 cm(-1) and D-o = 92cm(-1). They also indicate an excited CO(A(1)Pi)-Ar(1S) interaction with R-e = 3.55 Angstrom, D-e = 141 cm(-1) and D-o = 121 cm(-1). A red shift of 29 cm(-1) for the CO(X(1)Sigma)-A(S-1) --> CO(A(1)Pi)-A(S-1) vertical excitation energy, with respect to the corresponding CO(X(1)Sigma) --> CO(A(1)Pi) excitation in the absence of Ar, can be obtained by comparing the corresponding D-o values.