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

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Abstract
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 0 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.