In order to calculate the excess chemical potential of an adsorbed molecule onto the surface, I need to estimate the free energy by gradually decoupling it. To do so, I used the hybrid pair style to assign lj/cut/coul/long potential to the surface atoms and lj/cut/soft to the atoms in the molecule, and specified the required coefficients of each pair in the input file. To equilibrate the system within an NPT ensemble, I started with a lambda vaue of 1. However, I receive the error "Illegal pair style command". Could you please help me find out what the problem is?
When using only the lj/cut/coul/long potential, there is no such issue. So as an alternative approach, is it ok if I change the epsilon value of the test atoms manually in several runs (by factors of 0.0 to 1.0), and then do the subsequent calculations to obtain the free energy?
I'd truly appreciate your response.
Thanks in advance.