On the accuracy of <Emphasis Type="Italic">ab initio</Emphasis> calculations of absolute band intensities in the IR spectra of molecules

Absolute intensities are calculated in the harmonic approximation for the IR spectrum of 18 hydrocarbons, oxygen- and nitrogen-containing organic compounds. The quantum chemical calculation is carried out in the 6-311G(3df,3pd) basis set. The calculated data are compared to the experimental values of absolute absorption intensities. It is found that calculations in the HF approximation substantially overestimate (on average by 87%) the integral absolute intensity of fundamental vibrations in the 575–4000 cm^?1 range. Most part of this overestimation falls on the stretching vibrations with large amplitudes (C-H and C=O bonds). When the MP2 method is used without electron correlation treatment, this overestimation decreases to 22% and becomes more uniform in the whole spectral region.