DFT and statistical mechanics entropy calculations of diatomic and polyatomic molecules

摘要

Ab initio/DFT andstatistical mechanics methods for calculation of entropy are reported for 20 moleculesin which internal rotations are absent, and are compared with the experimentalvalues. Agreement between these data and corresponding entropies calculatedfrom spectral data is excellent, being within one entropy unit in almost allcases at 298.15 K. Entropies have also been calculated over a wide range oftemperature i.e., 298.15 K and 0–400 K at 50 K intervals. It is clearlyobserved that all entropies increase with increasing temperature. The resultsshow a sharp increase on going from 0 K to 50 K while the increase is slight attemperatures above 50 K. The results also suggest that such calculations mayassist in studies on entropies and equilibria involving species for whichvibrational frequencies can not easily be determined by experiment.