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The calculation of out-of-plane bending vibrations for 1T-bonded systems appears to be extraordinarily sensitive to the choice of a one-particle basis set. Ab initio predictions are reported for acetylene, an extreme example, at the self-consistent field (SCF), singles and doubles configuration interaction (CISD), nth order M011er-Plesset perturbation theory (MPn, n= 2-4), coupled-pair functional (CPF), and singles and doubles coupled cluster (CCSD) levels of theory. It is found that the addition of a set ofj basis functions to the carbon atom changes the value of the SCF 1T g frequency by+ 45 cm-I, and the value of all correlated 1T g frequencies by more than+ 100 cm-I. Evidence is presented that this behavior is present in other 1T-bonded systems. It is concluded that basis sets consisting of triple zeta plus two sets of polarization functions plus one set ofl functions (TZ2P+ I) can predict highly accurate (-1% average error …