General-model-space state-universal coupled-cluster method: excitation energies of water

A recently developed multireference (MR), general-model-space (GMS), state-universal (SU) coupled-cluster (CC) method [J. Chem. Phys. 119, 5320 (2003)] in its extended form that considers singly (S) and doubly (D) excited cluster amplitudes relative to the reference configurations spanning the model space-which can differ by up to and including quadruple excitations-is employed to investigate a number of low-lying excited states of the water molecule. The role played by different atomic orbital basis sets is examined by employing both the aug-cc-pVTZ and cc-pVTZ+diff bases, the latter extending the cc-pVTZ basis by an s diffuse function on hydrogen and two sets of s and p diffuse functions on oxygen. The computed vertical excitation energies are compared with those obtained earlier with a cc-pVDZ basis set, as well as with other-both theoretical and experimental results, when available. For a number of relevant states the equilibrium geometry, and the implied T-o excitation energies, are also given. It is shown that two sets of s and p diffuse functions on oxygen are essential for an accurate rendering of the experimental spectra and thus play a much more important role than the higher-angular-momentum diffuse functions (i.e. d and f on oxygen and p and d on hydrogen) characterizing the aug-cc-pVTZ basis set.