Calculate chemical potential using fep

Dear lammps-users,

I am trying to calculate the chemical potential of pure methane in NPT ensemble using lammps. My route is like this: The potential energy and volume purterbation of system delta_U and delta_V calculated by “fix adapt/fep” and “compute fep” commands firstly, and the chemical potential at fixed temperature and pressure could computed from formula mu=mu_ideal+mu_ex, mu_ex=-kTln(<Vexp(-betadelta_U)>/) . However, I got incorrect chemical. My input script has been attached, could you give me some suggestions about calculating chemcial potential. Many thanks!

Best regard.
Dongbo Wang

there was nothing attached, and even there was, it is unlikely that somebody will just edit it and do your work for you.
the process you want to do has been discussed in detail in publications and text books on free energy methods for molecular simulations and there are discussions on how to determine and minimize errors.

in general, the procedure to apply a new (to you) method is to first study provided examples and there are several included with LAMMPS and then extend those to similar problems with available reference results before moving on to your specific system of interest.
please also keep always in mind that you are looking at the properties of classical models, so a perfect match with experimental data is often coincidental.