Calculation and fitting of potential energy and dipole moment surfaces for the water molecule: Fully ab initio determination of vibrational transition energies and band intensities

Potential energy and dipole moment surfaces for the water molecule have been generated by multireference singles-and-doubles configuration interaction calculations using a large basis set of the averaged-atomic-natural-orbital type and a six-orbital-six-electron complete-active-space reference space. The surfaces are suitable for modeling vibrational transitions up to about $\text{11\hspace{0.17em}000\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-1}}$ above the ground state. A truncated singular-value decomposition method has been used to fit the surfaces. This fitting method is numerically stable and is a useful tool for examining the effectiveness of various fitting function forms in reproducing the calculated surface points and in extrapolating beyond these points. The fitted surfaces have been used for variational calculations of the 30 lowest band origins and the corresponding band intensities for transitions from the ground vibrational state. With a few exceptions, the results compare well with other calculations and with experimental data.