# How to calculate Gibbs free of a reaction?

I consider this reaction : NaOH (solid) + CO2 (gas) --> NaHCO3 (solid)

As far as I understand, Gibb free energy can be estimated like this:

Gibb = \delta(H) + \delta(ZPE) - T\delta(S)

\delta(H) = E(DFT_NaHCO3) - E(DFT_NaOH) - E(DFT_CO2)

because of solid phase, ZPE(NaHCO3) = ZPE(NaOH) = 0 and S(NaHCO3) = S(NaOH) = 0

\delta(ZPE) = ZPE(NaHCO3) - ZPE(CO2) - ZPE(NaOH) = -ZPE(CO2)

\delta(S) = S(NaHCO3) - S(CO2) - S(NaOH) = -S(CO2)

Overall, Gibb = E(DFT_NaHCO3) - E(DFT_NaOH) - E(DFT_CO2) - ZPE(CO2) + T * S(CO2)

Is this formula correct?

Meanwhile you have also approximated the enthalpy as the energy (where you miss the $$pV$$ correction), and neglected the entropy of solids. While you may justify the latter because you don't want to do phonon calculations on the solids (because of program or computational resource limitations, for example), I see no reason why you don't include the $$pV$$ correction - it's small but it's available for free.