For a crystal, once the total energies vs volumes was obtained by ATK, the data can be fitted to some formulas for the equation of state (e.g., Birch-Murnaghan 3rd-order equation of states) to obtain the enthalpy.
The DFT calculation in ATK is done for the 0K. For entropy (S), the effect of temperature needs to be considered. Usually, the vibration of lattice (i.e., the phonon) needs to be calculated. This can be done with ATK and other free code such as PHON (Dario Alfè,
http://www.homepages.ucl.ac.uk/~ucfbdxa/phon/). From the phonon spectrum of crystal, the free energy can be obtained by means of quasi-harmonic approximation.
