--As you may know, the formation energy of a hydrogenated graphene nanoribbon can be calculated using

E(f) =  ( E(ribbon) − n(C) · E(C) − n(H) · E(H2) / 2 ) / 2L

E(pristine ribbon) : total energy of hydrogenated nanoribbons  

E(H) and E(C): the total energy of free Hydrogen and CarbonL: the periodic length of the nanoribbon

--But this is for perfect vacuum conditions around freestanding graphene edges.

--In order to consider a molecular hydrogen gas atmosphere around the graphene edge (experimental conditions), the calculated total edge formation energy E(f) can be compared to the hydrogen chemical potential μ(H2) , resulting in the relative edge stability (Gibbs free energy):

 G(H2) = E(f) − ρ(H) · μ(H2) /2  ---->   Gibbs free energy 

where ρ(H) = n(H) / 2L ---->   Edge hydrogen density

μ(H2) = H0(T)- H0(0)- TS0(T)+ kBT ln (P/P0) ---- > Chemical Potential of H2 Molecule

 H: Enthalpy

S: Entropy

---Does anybody know how to calculate μ or chemical potential for H or halogens with above formula?