Ab initio potential-energy surface for the He(1S) plus NO(X 2Π) interaction and bound rovibrational states

Adiabatic potential-energy surfaces for the (2)A' and (2)A " states of the He(S-1)-NO(X (2)Pi) complex have been calculated at the restricted coupled cluster level of theory including single, double, and noniterated triple excitations [RCCSD(T)]. The potential-energy surface (PES) of the A' state has three minima: for the T-shaped geometry, barely skewed toward oxygen (R=6.07 a(0), Theta=96.7 degrees, and D(e)approximate to 29.2 cm(-1)), and for two collinear forms. In contrast, PES of the A " state has two minima, both related to T-shaped forms: (i) A global minimum, with He shifted toward nitrogen (R=6.27 a(0), Theta=76.1 degrees, and D(e)approximate to 25.3 cm(-1)), and a shoulderlike minimum, with He shifted toward oxygen (R=6.34 a(0), Theta=117.6 degrees, and D(e)approximate to 24.5 cm(-1)). The collinear forms of the A " state correspond to saddle points. The A' surface lies above the A " surface, except in the vicinity of perpendicular arrangement. The interaction energies have been analyzed via perturbation theory of intermolecular forces. The shape and location of the minima is determined primarily by the anisotropy of the exchange component, and stronger repulsion of A' is due to repulsion between the singly occupied anti-bonding pi* orbital and He. A variational calculation of the bound rovibrational states supported by the potential suggests that the HeNO complex is bound by about 7 cm(-1). The bound levels correlate with NO rotational levels with j=1/2 and j=3/2, and are well described as nearly free-rotor states in which j (NO rotation) and L (end-over-end rotation of R) are nearly good quantum numbers. Excited intermolecular stretching vibrations are not predicted to be bound. (C) 2000 American Institute of Physics. [S0021-9606(00)30404-4].