Dears,

I am a beginner in molecular dynamics and I need your help to solve this problem.

I am running a simulation using water/surfactant/decane/surfactant/water with 28438 atoms, 3ns NPT to shrink the box (50x50x120A), obtaining the right density and 2 ns NVT at 323K and 98 atm.

At NPT, the pressure is very good, near 100 atm without big fluctuations. But at NVT the pressure goes down, remaining in equilibrium in 50 atm.

My question is: how should I change the script to maintain the pressure close to 98 atm during NVT steps?

I tried do solve this problem reading the LAMMPS Manual and also here the Mailing Lists. But I was not able.

My Script:

pair_modify mix arithmetic tail yes

kspace_style pppm 1.0e-4

minimize 1.0e-4 1.0e-6 100 1000

reset_timestep 0

fix SHAKE all shake 0.0001 20 0 b 1 3 9 11 a 1

neighbor 2.0 bin

timestep 1.0

variable TK equal 323.15

variable PBAR equal 98.6923

velocity all create ${TK} 123456 rot yes mom yes dist gaussian

variable nsteps equal 2000000 # 2000 #

variable nptsteps equal 3000000 # 2000 #

variable nhalf equal {nsteps}/2
variable nprint equal {nsteps}/2000

variable ndump equal {nsteps}/100
variable xyzdump equal {nsteps}/20

variable padval equal 7 # 5

variable nrdf equal 100 # 10 #

variable Tdamp equal 100 # 10 #

variable Pdamp equal 500 # 50 #

# fix 1 all nvt temp {TK} {TK} ${Tdamp}

fix 1 all npt temp {TK} {TK} {Tdamp} iso {PBAR} {PBAR} {Pdamp}

variable xPress equal c_thermo_press[1]

variable yPress equal c_thermo_press[2]

variable zPress equal c_thermo_press[3]

variable IFT equal (0.5*lz*(v_zPress-0.5*(v_xPress+v_yPress))*0.0101325)

dump 2 all image {xyzdump} nvt.*.jpg type type zoom 1.0 adiam 1.5 size 720 1280 dump_modify 2 pad {padval}

thermo_style custom step pe ke etotal temp press density vol lx ly lz pxx &

pyy pzz pxy pxz pyz v_IFT

thermo ${nprint}

run {nptsteps} unfix 1 fix 2 all nvt temp {TK} {TK} {Tdamp}

# fix 2 all npt temp {TK} {TK} {Tdamp} iso {PBAR} {PBAR} {Pdamp}

run ${nsteps}

write_restart restart.*.lmp
write_data nvt.*.data

Thank you so much for your attention.

Kind regards,

Dr. Flávio Augusto de Freitas

Pós-doutorando na Curtin University

+61 426 814 028