Ab initio study for the intermolecular potential of the water–nitric oxide complex

Ab initio UMP2 gradient calculations confirm that there are a few stable structures of the complex H2O⋯NO (X 2Π). The A′ state has three minima: the N-bonded anti structure is the global minimum. Two other minima have also been located (N-bonded syn and O-bonded anti forms). By contrast, the A′′ has one minimum related to a T-shaped form, which lies above the A′ global minimum (UMP2 gradient optimisation procedure followed by calculations at the CCSD(T) level). The comparison of the IPES for the A′ and A′′ states calculated from a grid of points at the UMP2 level followed by the CCSD(T) calculations for the characteristic points on the IPES demonstrated the failure of the UMP2 calculations for the H2O⋯NO complex.