Dear all,

I am trying to calculate the free energy of hydration of ethanol in TIP4P water using the

USER FEP method. I have a problem with the TIP4P and ethanol LJ pair coefficients.

If I run the simulation letting LAMMPS pick the i-j coefficients with the mix geometric

command then the simulation runs perfectly fine. If I choose them explicitly then

I needed to play around with some labels to get a decent run otherwise the carbon atom

and the oxygen atom of the water got way too close.

pair_coeff 1 5 lj/cut/tip4p/long/soft 0.1244 3.3151 **1.0** #C_3 OW

Does anyone know why this happens and what is the correct way to choose the i-j **0.0 1.0**

coefficients as above.?

This is part of the input file:

pair_style hybrid &

lj/cut/coul/long 10.0 10.0 &

lj/cut/tip4p/long/soft 5 6 5 3 0.125 1 0.5 10.0 10.0 10.0

pair_modify tail no

kspace_style pppm/tip4p 1.0e-4