[lammps-users] what is proper way to calculate static pressure ?

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1

Dear All,

I am calculating stable crystal structure of SiO2 at certain temperature(for example 0) and pressure (for example 200). But pressure I obtained (1057) is far from desired pressure (200).

Is there a better way to calculate it?

units metal
atom_style charge
boundary p p p
read_data data.alfacristobalite
pair_style tersoff
pair_coeff * * /home/mauludi/lammps-6Apr09/lammps-6Apr09/potentials/SiO.tersoff Si O

velocity all zero linear

fix 1 all box/relax xyz 200.0 vmax 0.001

minimize 1.0e-4 1.0e-6 1000 1000

compute temperature all temp
compute coordinate all coord/atom 2.0
compute kineticenergy all ke
compute potentialenergy all pe
compute potentialperatom all pe/atom

thermo 10
thermo_style custom atoms step temp etotal pe ke ebond press
timestep 1
dump 1 all custom 100 dumpSiOalfacristo*.dat id type xs ys zs q
run 100

2

You should try rerunning another min - sometimes subsequent
iterations can get closer - you should also expt with other minimizer
settings, e.g. the line search - see the min_mofidy command.

Aidan, any comments on fix box/relax?

Steve

3

Minimization is always a tricky business. Possibly if you switch to from
backtrack to quadratic linesearch you will do better. Also, examine the
output from the minimization. The termination criterion will tell you why
the pressure is not fully converged. Finally, in your subsequent dynamics
run, you have set the timestep to 1 ps, which is not going to work very well
(if you actually use it to run MD).

Aidan