METHODS FOR THE CALCULATION OF NU-OH IN OH-O HYDROGEN-BONDS

Ab initio calculations are reported for dimerization-induced changes, DELTA-k, in the harmonic force constant k of the H-bonded OH in water dimer. Two dimer geometries are considered. DELTA-k is obtained by considering the perturbation of a given monomer OH potential by the interaction energy in the dimer in question. The interaction energy is partitioned to identify the role of the various contributions to DELTA-k. The sensitivity of DELTA-k to the choice of the one-electron basis set is studied by using five different basis sets, some of which have a set of bond functions in the H-O bond. At the correlated level, correction for basis set superposition error is found to be essential. A comparison is made of the correlation contribution to DELTA-k as given by the CEPA1, MP2, MP3, and MP4 methods. Of these, MP2 gives exaggerated results. Nevertheless, for economical and reasonably accurate calculations on large systems the MP2 approach in the ESPB basis set is advocated. The most accurate calculations yield a shift DELTA-nu-0-1 of - 121 cm-1 for the uncoupled donor O-H vibrational frequency in water dimer.