DFT and statistical mechanics entropy calculations of diatomic and polyatomic molecules

Ab initio/DFT and statistical mechanics methods for calculation of entropy are,reported for 20 molecules in which internal rotations are absent, and are compared with the experimental values. Agreement between these data and corresponding entropies calculated from spectral data is excellent, being within one entropy unit in almost all cases at 298.15 K. Entropies have also been calculated over a wide range of temperature i.e., 298.15 K and 0-400 K at 50 K intervals. It is clearly observed that all entropies increase with increasing temperature. The results show a sharp increase on going from 0 K to 50 K while the increase is slight at temperatures above 50 K. The results also suggest that such calculations may assist in studies on entropies and equilibria involving species for which vibrational frequencies can not easily be determined by experiment.