F of the harmonic solid
Ftotal(V,T) = Fperfect(V,T)+Fvibrational(V,T)
Fperfect from static electronic minimisation calculation including thermal electronic excitations via:
Fperfect (V,T) = U0(V)+ Uel(V,T) -TSel(V,T)
Fvibrational requires _i and vibrational DOS to put into statistical mechanics equations.