lammps-users Digest, Vol 150, Issue 10

The soft pair styles are important for the sites that are being created/deleted during your free energy route, otherwise there will be a discontinuity.
For other sites in your system that are not directly affected or don’t go to zero, then normal pair styles can be used.
In your question you don’t provide details about what species are involved in the free energy calculation for a more precise answer.

Reading the doc pages for the pair styles and looking at the examples distributed with the source code will certainly help.
Also, trying a simple test system first is a good idea.

Hope this helps,
Agilio

Thanks for your reply.

I want to calculate water free energy.
So I need to use soft core potential for one water molecule with rest of it !
This is the input file :
( I choose one TIP4P water molecule with it’s dummy-site and used pair_style lj/cut/coul/long/soft for this molecule interaction with rest of water molecules)

pair_style hybrid lj/cut/coul/long/soft 1.0 0.5 10.0 12.0 8.5 &
lj/cut/tip4p/cut 2 1 1 1 0.15 12. 8.5
kspace_style pppm 1.0e-4
pair_modify tail no

pair_coeff 1 1 lj/cut/tip4p/cut 0.0000 1.0000 # HW-OW
pair_coeff 1 2 lj/cut/tip4p/cut 0.0000 1.0000 # OW-HW
pair_coeff 2 2 lj/cut/tip4p/cut 0.1852 3.1589 # OW-OW

pair_coeff 1 3 lj/cut/coul/long/soft 0.0000 1.0000 0.0 # HW-HW
pair_coeff 1 4 lj/cut/coul/long/soft 0.0000 1.0000 0.0 # HW-OW
pair_coeff 1 5 lj/cut/coul/long/soft 0.0000 1.0000 0.0 # HW-M
pair_coeff 2 3 lj/cut/coul/long/soft 0.0000 1.0000 0.0 # OW-HW
pair_coeff 2 4 lj/cut/coul/long/soft 0.1852 3.1589 0.0 # OW-OW
pair_coeff 2 5 lj/cut/coul/long/soft 0.0000 1.0000 0.0 # HW-M

pair_coeff 3 3 lj/cut/tip4p/cut 0.0000 0.0000 # HW-HW
pair_coeff 3 4 lj/cut/tip4p/cut 0.0000 0.0000 # HW-OW
pair_coeff 4 4 lj/cut/tip4p/cut 0.1852 3.1589 # OW-OW
pair_coeff 5 5 lj/cut/tip4p/cut 0.0000 0.0000 # HW-OW

bond_coeff 1 450 0.9572
angle_coeff 1 55 104.52
bond_coeff 2 450 0.9572
angle_coeff 2 55 104.52

group WOH1 type 1 2
group WOH2 type 3 4
group WOHM type 3 4 5

set type 1 charge 0.5564
set type 2 charge -1.1128
set type 3 charge 0.5564
set type 5 charge -1.1128