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[lammps-users] Free energy calculation using TI method
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[lammps-users] Free energy calculation using TI method


From: Manish Maurya <manishmm@...71...>
Date: Mon, 4 Dec 2017 17:45:43 +0530

Dear LAMMPS users,

I am trying to calculate free energy using thermodynamic integration algorithm implemented in LAMMPS. I have got the results for free energy but I am not getting correct trend in free energy with respect to coupling parameter in fact I am getting opposite results. I mean to say, at low value of lambda (0.01) magnitude of free energy should be less as compared to high value of lambda (1). So, I want to know where am I making mistakes? I am writing my input script and calculated free energy data.

#LAMMPS (11 JAN 2012)

dimension   3

units       real

boundary    p p p

atom_style   full    

pair_style     lj/cut 15.0

read_data        full.data 

set type 1 charge 0.0

set type 2 charge 0.0 

pair_coeff      3 3  0.0556417148  3.4

pair_coeff      1 1  0.05366216    2.80

pair_coeff      2 2  0.15701151    3.05

pair_coeff      1 3  0.090333873   3.395

pair_coeff      2 3  0.093461816   3.225

bond_style       harmonic

bond_coeff     1  1000.00        1.16

angle_style      harmonic

angle_coeff    1  1000.00        180.0

group flakes      type 3

group so2         type 1 2

 neighbor      3.0 bin

neigh_modify     delay 1 every 1 check yes

neigh_modify    exclude molecule flakes

  variable        a loop  100

  label         loop

  variable       scale1  equal  ((0.0)+($a*0.01))

fix               5  flakes setforce 0.0 0.0 0.0

fix             6 so2 nvt temp 303.0 303.0 100.0

 variable        lj equal  v_scale1

variable        dlj equal 1

fix             eps1 all adapt 100 pair lj/cut epsilon 1 3 v_scale1

fix             eps2 all adapt 100 pair lj/cut epsilon 2 3 v_scale1

compute   ThermoInt  all ti lj/cut 1*3 v_lj v_dlj

fix    12 all ave/time 100 50 100000 c_ThermoInt

timestep      1.0

thermo        100000

#thermo_style      custom step temp press vol pe ke etotal

thermo_style    custom step v_scale1  c_ThermoInt

thermo_modify   flush yes

restart          100000 wtr.restart

dump     10 all custom 10000 wtr.lammpstrj id mol type x y z

dump_modify 10 sort id

run        100000

unfix           5

unfix           6

unfix eps1

unfix eps2

uncompute   ThermoInt

undump          10 

    next           a

    jump           in.dat loop


                  lambda               ThermoInt 

0.01

-259.56278

0.109

-47.054257

0.208

-26.47784

0.307

-14.863764

0.406

-14.22882

0.505

-8.4769589

0.604

-11.954373

0.703

-7.5411861

0.802

-8.8976038

0.901

-8.2287353

1

-7.6737742


Thank you in advance for your help.

Regards,


Manish Maurya
Research Scholar
Computational Nano-science Group
Department of Chemical Engineering
Indian Institute of Technology, Kanpur
Kanpur, UP, India -208016