Can density functional theory describe the NO(X2Π)-Ar and NO(A2Σ+)-Ar van der Waals complexes?

The interaction of nitric oxide (NO) in its ground state X(2)Π and the first excited Rydberg state A(2)Σ(+) with an argon (Ar) atom has been studied using density functional theory. A number of exchange-correlation functionals that account for dispersion interactions have been considered, including functionals with both empirical and non-empirical treatments of dispersion. To study NO in the excited state, the recently developed maximum overlap method was used. Potential energy surfaces for interaction of NO with Ar have been constructed and parameters describing their minima, such as NO-Ar distance, orientation angle, and binding energy, have been determined. A comparison with combined experimental and accurate theoretical data has been made in terms of these parameters and the overall shape of the surfaces. For the ground state, several of the functionals give very good results. Treatment of the excited state is more problematic. None of the functionals considered provides completely satisfactory results. Several reasons for this failure have been identified: an incorrect description of the non-dispersion component of the interaction and the damping of the dispersion interaction at small interatomic distances.