Ab initio UMP2 gradient calculations confirm that there are a few stable structures of the complex H2O . . . NO ((XII)-I-2). 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.