Impact of local approximations on MP2 vibrational frequencies

Equilibrium structures and harmonic vibrational frequencies have been computed using local and conventional second-order Moller-Plesset perturbation theory (MP2) for 16 small and medium size molecules (up to 14 atoms). It is found that the deviations caused by the local approximation are usually negligible, and that the quality of the results relative to experiment is essentially identical for both methods. Notable deviations (of the order of 20 cm(-1)) are only observed in cases where two vibrational modes are strongly coupled and are therefore extremely sensitive to small changes of the force constants. The deviations between LMP2 and MP2 decrease with increasing basis set, and the basis set dependence is more pronounced for LMP2. This indicates that intramolecular basis set superposition errors are reduced in the local correlation treatment. (C) 1999 Elsevier Science B.V. All rights reserved.