Gibbs Free Energies

For the calculation of equilibria or rate constants of reactions, Gibbs free energies of intermediates or transition states need to be calculated. With the textbook harmonic osciallator/rigid rotor approximation at hand, this essentially is reduced to performing a single frequency calculation of the system (see here) for details.

The following calculation of a free energy cannot be done by VASP itself. Instead, the VASPGibbs python script can be used. It can be obtained from the following github page.

The application is quite simple: First, do a freqency calculation of the structure with the usual settings, with or without selective dynamics is possible.

Then, the analysis can be started with:

vasp_gibbs -t [temperature]

where the temperature, at which the reaction of interest takes place, must be given (e.g.; -t 300 for 300 K). The results of the analysis are written to the file VaspGibbs.md, which is well documented and self-contained.