On the reliability of SCF ab initio calculations of vibrational frequencies and intensities of hydrogen-bonded systems

Abstract

A critical analysis is presented of a variety of popular basis sets with regard to the SCF calculated harmonic frequencies and intensities in (HF) 2 and (H 2O) 2. Both complexes pose interesting tests because large basis set calculations are available for comparison, as are a few experimental data. The computed shifts of the intramolecular vibration frequencies upon dimerization are in plausible agreement with experiment for almost all basis sets considered. Exceptions are the 3-21G basis set which greatly overestimates red shifts and the MINI-1 set which underestimates them for (HF) 2. For the water dimer neither basis set predicts a qualitatively correct order of frequency shifts. The intermolecular vibrational frequencies, the intensities and changes of intensities upon complex formation are found to be very sensitive to the quality of basis set. Among the minimal and split-valence types the best agreement with large basis set calculations is noted for the 4-31G basis set.