Hi,

I’ve been getting to grips with Lammps and I’ve been trying to minimize a unit cell of alpha quartz under constant pressure to obtain optimum lattice parameters and atom coordinates. I am using the Lammps code released on 9th February 2011. I have tried various minimization techniques and using a barostat but to no avail. My input files are along the lines…

# Calculation of properties of alpha quartz using BKS potential and minimization

units metal

newton on

atom_style charge

dimension 3

boundary p p p

lattice custom 5.4054 origin 0.0 0.0 0.0 &

a1 0.90950000 0.00000000 .00000000 &

a2 0.00000000 0.78760000 .00000000 &

a3 .00000000 .00000000 1.00000000 &

basis 0.46990000 0.00000000 0.66666667 &

basis 0.00000000 0.46990000 0.33333333 &

basis 0.53010000 0.53010000 0.00000000 &

basis 0.41410000 0.26810000 0.78540000 &

basis 0.73190000 0.14600000 0.45206667 &

basis 0.85390000 0.58589999 0.11873333 &

basis 0.26810000 0.41410000 0.21460000 &

basis 0.58589999 0.85399999 0.88127777 &

basis 0.14600000 0.73190000 0.54793333

region box prism 0 1 0 1 0 1 -0.4547 0 0

create_box 2 box

create_atoms 1 box &

basis 1 1 &

basis 2 1 &

basis 3 1 &

basis 4 2 &

basis 5 2 &

basis 6 2 &

basis 7 2 &

basis 8 2 &

basis 9 2

mass 1 28.09

mass 2 16.0

group siliconatoms type 1

group oxygenatoms type 2

set group siliconatoms charge 2.4

set group oxygenatoms charge -1.2

kspace_style ewald/n 1.0e-4

pair_style buck/coul/long 10.0

pair_coeff 1 1 0.0 0.0001 0.0

pair_coeff 1 2 18003.7572 0.205205 133.5381

pair_coeff 2 2 1338.7730 0.362319 175.0000

variable density equal “(mass(all)*1.660538782) / (vol)”

thermo 1

thermo_style custom step temp etotal pe evdwl ecoul press vol lx ly lz v_density xy yz xz

dump 1 all xyz 1 alpha_quartz_bks.xyz

fix 1 all box/relax tri 0.0

#min_modify line quadratic

minimize 0 1.0e-1000 1000 1000

min_style hftn

minimize 0 1.0e-100 100000 100000

The minimizations give me a density of 2.75 g/cm^3 when it should be 2.65g/cm^3. The energies I get, however, are similar to the results I get from minimizations using BKS in other codes. I get -172.26397eV from Lammps and -175eV from other codes. I have tried minimizing with constant volume then, minimizing with fix box/relax and my results are getting much closer (energy:-174eV, density: 2.68g/cm^3). I was wondering if anyone has any reccomnmendations for doing constant pressure minimizations and could help me get the results I need.

Many thanks,

Tassem El-Sayed