Hello,
I am recently working on an alchemical transformation that involves turning off coulombic and LJ interactions sequentially between a cation and a framework. I have been setting up the input according to the fep/CH4hyd/fep10 example in LAMMPS. I wonder why it is necessary to scale down the charges while the coul lambda value is being scaled down as shown below in the first example scripts.
In a second example, fep/SCPEhyd, the decoupling was done in two steps: q and LJ sequentially. In this case, however, only the charges are being scaled down while the coul/long pair-wise interactions are not changed. Could anyone justify the differences between the two cases?
Thanks a lot !!
Best,
Jianming
fep10/CH4hyd
fix ADAPT all adapt/fep 100000 &
pair lj/cut/tip4p/long/soft lambda 1*2 3*4 v_lambda &
atom charge 1 v_q1 &
atom charge 2 v_q2 &
after yes
fix PRINT all print 100000 "adapt lambda = ${lambda} q1 = ${q1} q2 = ${q2}"
variable dlambda equal -0.05
variable dq1 equal -0.24*v_dlambda
variable dq2 equal 0.06*v_dlambda
compute FEP all fep ${TK} &
pair lj/cut/tip4p/long/soft lambda 1*2 3*4 v_dlambda &
atom charge 1 v_dq1 &
atom charge 2 v_dq2
fix FEP all ave/time 20 4000 100000 c_FEP[1] c_FEP[2] file fep10.lmp
fep/SPCEhyd example
fix ADAPT all adapt/fep 100000 &
atom charge 1 v_qH &
atom charge 2 v_qO &
after yes
thermo_style custom step etotal ke pe evdwl ecoul elong temp press density v_lambda v_qO v_qH
variable dlambda equal -0.05
variable dqH equal 0.4238*v_dlambda
variable dqO equal -0.8476*v_dlambda
compute FEP all fep ${TK} &
atom charge 1 v_dqH &
atom charge 2 v_dqO &
volume yes