[lammps-users] annealing a grain boundary using the fix npt command

Hi all,

I'm trying to anneal a grain boundary in silicon. My annealing
"protocol" is as follows: using a constant (zero) pressure ensemble
that allows the x, y, and z direction allow to change independently, I
equilibrate the system at T = 300 K, 600 K, 900 K, and 1200 K for
50,000 md steps each. I then decrease the temperature continuously
from 1200 to 300 K over 1 million MD steps

For step 1, I use the following npt fix:
fix 1 all npt temp 300.0 300.0 .1 aniso 0.0 0.0 1000.0

and it returns this error:
ERROR: Illegal fix npt command

What am I doing wrong in my fix?

Zack Cordero

1st half of my input script:
#Annealing grain boundaries in Silicon

boundary p p p
units metal
echo screen
newton on
log log.si_anneal

#create atoms
atom_style atomic
read_data position1.xyz

group all type 1
pair_style tersoff
pair_coeff * * Si.tersoff Si
neighbor 2.0 bin
neigh_modify every 1
dump 1 all xyz 100 position.xyz
thermo 100

velocity all create 400 429349 dist gaussian
fix 1 all npt temp 300.0 300.0 0.1 aniso 0.0 0.0 1000.0

timestep .01

run 50000

Dear Zach,
fix npt syntax has changed recently. Please see the doc page.

Thanks for replying. I triple-checked here
(http://lammps.sandia.gov/doc/fix_nh.html) before I e-mailed out to
the list.

According to the example on the doc page the input should go like
fix 1 water npt temp 300.0 300.0 100.0 iso 0.0 0.0 1000.0

The only difference between that one and mine is that I changed the
Tdamp to .1 and the iso to aniso.

Am I missing something?


Dear Zack, when you use aniso, you have to to starting and ending pressure in all three directions as they are being barostated independently. So, try this out
fix 1 water npt temp 300.0 300.0 100.0 aniso 0.0 0.0 0.0 0.0 0.0 0.0 1000.0

The WWW page docs refer to the most current version
of LAMMPS. Are you running the most current version
of LAMMPS? The syntax for fix npt changed in April,
as I recall (see the bug page).