Just calculate the difference of the total energies

E(adsorbat/surface) - (E(adsorbat)+E(surface))

where you calculate the three energies with CASTEP or any other code.

I agree with Viorel Chihaia

Thanks Mr. Viorel Chihaia for valuable answer.

as indicated by Viorel Chihaia i would like just to add :

if you find an adsoption energy less than about 10 kcal/mol you have a physical adsorption character.

if you find an adsoption energy more than about 10 kcal/mol you have a chemical adsorption character.

I agree with Viorel Chihaia and Mohamed Sidoumou