Use of DFT Methods for the Calculation of the Entropy of Gas Phase Organic Molecules: An Examination of the Quality of Results from a Simple Approach

Entropy values for organic molecules can be calculated with useful accuracy using Gaussian 94 at the B3LYP/6-31G** level, provided the values are corrected for the number of low lying conformations by adding an entropy of mixing term. This has been tested by calculations on 128 organic compounds, with particular focus on relatively large molecules with up to 10 carbon atoms. The overall standard deviation is 1.28 (cal/K)/mol in entropy, equivalent to 0.38 kcal/mol in free energy at 25 °C, with only seven deviations greater than two standard deviations and only two greater than three standard deviations.