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[lammps-users] compute ti command
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[lammps-users] compute ti command


From: Amir Taghavi <amir_taghavi14@...16...>
Date: Fri, 4 May 2018 18:35:43 +0000 (UTC)

Hello,

I have been trying to test a few free energy methods to calculate the solvation free energy of a LJ solute (B) in the same LJ solvent (A),  a very simple model system.
These methods include free energy perturbation (FEP), Widom insertion, and thermodynamic integration (TI) methods.

The results I've got are as following:
FEP:   -3.61 (0.05)
Widom:             -3.78 (0.2)
TI:  -4.22 (0.02)

I can't see why TI method doesn't give the right answer. Here is my input file:

***begin***
units lj
dimension 3
boundary p p p
atom_style atomic
read_restart run_b.rest

pair_style hybrid    lj/cut 2.5  lj/cut/soft 1.0 0.5 2.5 
mass   1      1.0000 # A
mass   2      1.0000 # B
pair_coeff  1     1    lj/cut            1.0 1.0 # A    A
pair_coeff  1     2     lj/cut/soft 1.0 1.0  ${Lambda} # A    B 
pair_coeff  2     2    lj/cut            1.0 1.0 # B    B 

restart         10000   run_1.rest run_2.rest


thermo 10000
timestep        0.01
run_style       verlet

variable lambda equal 1
variable           dlambda equal 1


compute TI all  ti lj/cut/soft   1*2   v_lambda v_dlambda

fix   NPT all npt temp 0.8 0.8 1.0  iso 0.1  0.1 10.0 
run 300000

reset_timestep 0
fix thermodynamic all ave/time 1 100 100   c_TI    file   Thermodynamic.dat

run 1000000
***end***

Please note that I am doing separate independent simulations in a parallel way by having ${Lambda} from 0 to 1.0 with 0.025 intervals (41 simulations). 
I'm using lammps 16Mar18. 
Any thoughts will be appreciated.