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I used VASP for quite a lot and I have thought that total energy in OUTCAR is equal to Gibbs free energy

because static DFT calculation is done at 0K and no strain is applied to the crystal structure,

so Gibbs free energy G = (internal energy) = (total energy).

However, recently I doubt this is correct.

I heard that we should also calculate phonon frequency things. Am I right?

Thanks for reading.

Yes, you are. ;-)

There are a lot of papers around, which deal with that in detail. Best is, to pull one which covers your field, e.g. semiconductors, ..., and with some detailed description.

You'll also look into a code called 'phonopy', also listed as tool on the VASP website.

Cheers,

alex