Abstract
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.
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