A New Full-Dimensional Ab Initio Intermolecular Potential Energy Surface and Rovibrational Energies of the H 2 O-H 2 Complex.

H 2 O-H 2 is a prototypical five-atom van der Waals system, and the interaction between H 2 O and H 2 plays an important role in many physical and chemical environments. However, previous full-dimensional intermolecular potential energy surfaces (IPESs) cannot accurately describe the H 2 O-H 2 interaction in the repulsive or van der Waals minimum region. In this work, we constructed a full-dimensional IPES for the title system with a small root-mean-square error of 0.252 cm -1 by using the permutation invariant polynomial neural network method. The ab initio calculations were performed by employing the explicitly corrected coupled cluster [CCSD(T)-F12a] method with the augmented correlation-consistent polarized valence quintuple-ζ basis set. Based on the newly developed IPES, the bound states of the H 2 O-H 2 complex were calculated within the rigid-rotor approximation. The transition frequencies and band origins agreed well with the experimental values [Weida, M. J.; Nesbitt, D. J. J. Chem. Phys. 1999 , 110, 156-167] with errors less than 0.1 cm -1 for most transitions. Those results demonstrate the high accuracy of our new IPES, which would build a solid foundation for the collisional dynamics of H 2 O-H 2 at low temperatures.