[lammps-users] FDTI compute fep free energy calculation

Dear All,

I’m finding out the free energy of CO2 in oil. I’m using compute fep in LAMMPS and using the FDTI method with perturbation.

I have a question about the calculation of the integral during post-processing.

I change lambda values in steps of 0.25 (this is just for representation, in actuality I’m using a step size of 0.05) with a perturbation of -0.002 in case FDTI 1–>0 and 0.002 in case FDTI 0–>1. As in the tables below, I see that I’m not integrating through the entire range. For FDTI 1–>0, I am missing lower lambda values close to 0. For FDTI 0–>1, I am missing upper lambda values close to 1. Considering this, the absolute value of integral in both cases turn out to be different. What would be the solution so that the entire range is covered for free energy calculation? Or is there some issue with my understanding?

FDTI 1–>0
lambda 1 0.75 0.5 0.25
perturbed value 0.998 0.748 0.498 0.248

FDTI 0–>1
lambda 0 0.25 0.5 0.75
perturbed value 0.002 0.252 0.502 0.752

Thank you very much.

LAMMPS version: Aug2018

Kunal Mavani