Can anyone tell me how to calculate free energy change (Gibbs free energy) during phase-transition using GAUSSIAN.
You will have to do a frequency calculation. In the result log file you will find the Gibbs free energy and other thermodynamic properties such as enthalpy, entropy etc.
@ Xingyong Wang: Thank you Wang for your reply. I had asked same question to the Gaussian community also. They told me that it is not possible. According to them Gaussian deals with methods which describe the internal energy of a single molecule in a variety of environments. Where as, phase transition involves intermolecular forces of a group of molecules.
Maybe if you use a different functional that take into account intermolecular forces can help you using a freq calculation
Free Energy is answer of molecular interactions, where the effect is mostly derived from electrostatic forces, Hellmann-Feynman Theorem. But the main problem is "during phase-transition", I think it is not a equilibrium state, so to estimate assembled in this conditions is a rare event and the population is small and will produce large errors.
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