Calculated stretching overtone levels and Darling-Dennison resonances in water: a triumph of simple theoretical approaches.

The coupled-cluster singles and doubles treatment with a perturbative treatment of triple excitations known as CCSD(T) has been used in conjunction with a hierarchy of atomic natural orbital basis sets to study stretching levels in water up to 2 eV above the zero-point level. Agreement with experiment obtained with perturbation theory augmented by a simple treatment of resonances suggested by Lehmann is quite remarkable in spite of the well-known and strong Darling-Dennison resonances in this system. With the largest basis sets, deviation from experiment is ca. 20 cm-1 at energies in the 15,000-16,000 cm-1 range. [ABSTRACT FROM AUTHOR]