fep & system potential

Hi.

I use pair_style lj/cut/coul/long/soft for free energy calculation by FEP method.
My system’s potential when i use soft pair_style change suddenly while without it , the potential is the expected amount.
Can you please let me know what is the reason of this different amount of systems potential ?

This is the pair_style that I use for FEP calculation :

pair_style hybrid lj/cut/coul/long 12.0 8.5 &
lj/cut/coul/long/soft 2.0 0.5 10.0 12.0 8.5 &
lj/cut/tip4p/cut 2 1 1 1 0.1546 12. 8.5
kspace_style pppm 1.0e-4

Thanks a lot for your help in advance .