[lammps-users] Syntax error with NPT fix

Hi Ahmed,

Thanks for responding. Both commands below work just fine...

fix 1 all npt temp 300.0 300.0 100.0 iso 900.0 90000.0 1000.0
fix 1 all npt temp 300.0 300.0 100.0 aniso 900.0 90000.0 1000.0

...but the problem I'm having is that they appear to be different commands that can only produce the same result, in this case 90 Kbars of hydrostatic pressure. I actually don't want NPT, I want N (sigma)T. I thought aniso modifies fix NPT to carry out constant stress....

For iso, I expect the syntax to look as it does as it allows the user to specify initial and final hydrostatic pressures.

However for aniso, anisotropic, I thought I could specify a different stress values along along xx, yy or zz.
For example, if I want to find the tensile strength of a nanowire oriented along the z axis of my MD cell, why can't I use aniso to specify the zz component in the stress tensor with the following command?

fix 1 all npt temp 300.0 300.0 100.0 aniso z 900.0 90000.0 1000.0

This again returns an error, as you pointed out. I want to extend my system in one traverse direction until it fractures. How do I carry out the kind of tensile stress test I described in LAMMPS using constant stress MD?

Thanks again my friend,

Nicholas Jabari Lee

However for aniso, anisotropic, I thought I could specify a different stress values along along xx, yy or zz.
For example, if I want to find the tensile strength of a nanowire oriented along the z axis of my MD cell, why can't I use aniso to specify the zz component in the stress tensor with the following command?

as the documentation says, the aniso keyword is a shortcut notation for
having the _same_ target pressure, but having dimensions change independently
(couple none). iso, in turn has couple xyz, i.e. all change by the same amount.

if you do want different target pressures in each dimension, just don't use
the aniso keyword and specify the individual components.

cheers,
     axel.

Axel is correct. And aniso and iso do not do the same things. The former
treats the 3 dims independently (with the same target pressure). The latter
couples all 3 together, so the box shape stays the same.

Steve