The potential energy surface (PES) Of O-2-H2O was studied by second-order Moller-Plesset perturbation and multi-reference single-and double-excitation configuration interaction (MRSDCI) methods with aug-cc-pVTZ basis sets. The in-plane mutual rotations of the monomers and the intermolecular stretching were also investigated by numerically solving the Schrodinger equation for nuclear motions with the three-dimensional finite element method. The lowest states for the conrotatory motion are actually degenerate, while those for the disrotatory one are split by 6.9 cm(-1), which is consistent with the recent result of Fourier-transform microwave spectroscopy, where the C-2v structure was obtained. (C) 2004 Elsevier B.V. All rights reserved.