How to calculate Gibbs energy (G) from a frequency output file?
This is printed (at T = 298 K) at the end of a vibrational frequency job, e.g. for H2O molecule at the HF/STO-3G level I get:
The Molecule is an Asymmetric Top Translational Enthalpy: 0.889 kcal/mol Rotational Enthalpy: 0.889 kcal/mol Vibrational Enthalpy: 15.297 kcal/mol gas constant (RT): 0.592 kcal/mol Translational Entropy: 34.608 cal/mol.K Rotational Entropy: 10.673 cal/mol.K Vibrational Entropy: 0.001 cal/mol.K Total Enthalpy: 17.667 kcal/mol Total Entropy: 45.281 cal/mol.K
(You can tell it’s T=298 K based on the value of RT.) The value of G at T=298 K is directly computable from the values of H and S that are provided. If you’d like G at a different temperature, then you need to recompute U, H, and S based on harmonic partition functions from the vibrational frequencies that are given.