Minimization using the fix box/relax

LAMMPS LAMMPS Beginners
1

Dear Sir,

I have a question regarding using the fix box/relax command. Based on the LAMMPS documentation, it is suggested to apply a pressure or stress tensor during energy minimization to expand/shrink box to make the system getting to the minimum energy state. If I use the minimize command and while the temperature is zero, means zero contribution of dynamic pressure and the pressure is only virial.
Under this situation, what is the value of the pressure to be applied using fix box/relax?
Is it zero? If I apply a negative pressure to expand a box, will this be valid as I am still getting near zero viral pressure for the minimum energy state.

Best.

2

You need to provide more details about your system and the input your are using if you want some meaningful advice.

3

Thank you for the response.
I am running the case of bi-crystal in the online tutorial of lammps " LAMMPS Relaxed Bi-layer".
If I use the online model without changing anything, it is running for 33 mins. on my PC without reaching a near zero pressure, last value of 3233.6087 at step 685.
However, if I use an updated version, which is attached bellow the virial pressure reaches around 39 at the step of 106 of the second minimization, overall time is less than 5 min.
I have used two minimization commands: the first one runs just for 50 steps without using box/relax command.
The second minimization step uses fix/relax with pressure:
fix 1 all box/relax z -100000
minimize 1e-15 1e-15 106 5000
My concern is about applying a pressure of -100000, will this make any issues with the physical results or in another way, will the system of next case be in the minimum energy state while having the virial close to zero.

Best Regards.

Input file for bi-layered crystal, Nickle and Aluminum, relaxed

Tikelia Wright, July 2012

------------------------ INITIALIZATION ----------------------

clear
units metal
dimension 3
boundary p p p
atom_style atomic

----------------------- ATOM DEFINITION -----------------------

lattice fcc 3.52
region upper block 0 54 0 54 0 7
lattice fcc 4.05
region lower block 0 47 0 47 6.5 13
region whole union 2 upper lower

create_box 2 whole
lattice fcc 3.52 orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
create_atoms 1 region upper
lattice fcc 4.05 origin 0 0 .247 orient x 1 0 0 orient y 0 1 0 orient z 0 0 1
create_atoms 2 region lower

------------------------ FORCE FIELDS -----------------------

pair_style eam/alloy
pair_coeff * * Mishin-Ni-Al-2009.eam.alloy Ni Al

#---------------------------Settings----------------------------
compute csym all centro/atom fcc
compute eng all pe/atom
compute eatoms all reduce sum c_eng
compute 2 all stress/atom NULL

#----------------------Run Minimization-------------------------
reset_timestep 0
thermo 10
thermo_style custom step pe lx ly lz press pxx pyy pzz c_eatoms
dump 1 all custom 10 dump.comp.* id type xs ys zs c_csym c_eng c_2[1] c_2[2] c_2[3]
min_style cg

minimize 1e-15 1e-15 50 5000

######################################
undump 1

#----------------------Run Minimization-------------------------
reset_timestep 0
thermo 1
thermo_style custom step pe lx ly lz press pxx pyy pzz c_eatoms
dump 1 all custom 10 dump.comp.* id type xs ys zs c_csym c_eng c_2[1] c_2[2] c_2[3]
min_style cg

fix 1 all box/relax z -100000

minimize 1e-15 1e-15 106 5000

######################################

SIMULATION DONE

print “All done”

4

I am using this version of Lammps:
LAMMPS (7 Jan 2022)

5

Hi there!

I am wondering if you ever found a solution to this question? I am starting my journey with LAMMPS and I am trying to figure out how to relax my crystal structure before running an NVT. I would highly appreciate your insights.

Thanks in advance :slight_smile: