In Lammps Manual (v13Apr 2017), the fix gcmc command, the Syntax section, it said that:
“mu = chemical potential of the ideal gas reservoir (energy units)”
Then, in the Description section , it said this :
“ The chemical potential is a user-specified input parameter defined as:
μ=μ_id + μ_ex ”
(1)The Question is : does “chemical potential of the ideal gas reservoir” mean: the chemical potential of the “ideal gas” reservoir, or the “ideal” gas reservoir ? Which value I’m supposed to choose: μ,μ_id, or μ_ex ?
Through Widom’s Test Particle Insertion method, I got a value of = -23.33 KJ/mol of water, which is comparable to the excess chemical potential of -22.8 KJ/mol in Agarwal et al.(2014) ’s work.
Corti (1998)  also stated that:
“Widom [1, 2] showed that the excess chemical potential μex of a system of N interacting particles is given by
μex = - kT ln< exp(- φ/kT)>N”
So the Question is : from Widom’s Test Particle Insertion method, I got the excess chemical potential, could this excess chemical potential be used in ficx gcmc represent the mu value ?
 Agarwal, A.; Wang, H.; Schütte, C.; Site, L. D. Chemical potential of liquids and mixtures via adaptive resolution simulation. The Journal of chemical physics 2014, 141 (3), 034102.
 Corti, D. S. Alternative derivation of Widom’s test particle insertion method using the small system grand canonical ensemble. Molecular Physics 1998, 93 (3), 417-420.