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Hello

As the Gibbs free energy equation shows:

G = H - TS = U + PV -TS = E_DFT + E_ZPE + nRT -TS

I have always had a question. Since VASP is based on density functional theory, the calculated "U" of the electronic system should be in the ground state of the system, which means T = 0? According to the third law of thermodynamics, when T=0k, the entropy of the system should be 0; From this point of view, can we not use VASP to calculate the Gibbs free energy of a non-zero temperature system?

Hello Jun,

yes, we can. But it's not straightforward. I find the explanation of in the Gaussian whitepaper very instructive: http://gaussian.com/thermo/ You need to take into account tht the degrees of freedom change for solids/surfaces wrt. molecules.

One other possibility is MD of course.

Cheers,

alex