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Re: [lammps-users] how to apply loading with constant stress ratio
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Re: [lammps-users] how to apply loading with constant stress ratio


From: Steve Plimpton <sjplimp@...24...>
Date: Wed, 2 May 2018 08:53:32 -0600

Not clear on what you are asking.

Fix deform changes the box size/shape in a prescribed way
over time.  If you use its variable option you have a lot
of flexibility in what the x,y,z dimesions (e.g. of an otrhogonal
box) would be over time.

But if you are asking to adjust the box size based on the
measured stress components Pxx, Pyy, Pzz, then that
is barostatting, not fix deform,  So you need to look
at fix npt and its options.  I don't think it can do what
you are asking however.

We could possibly add a variable option to fix npt
to allow the target pressure (e.g. in x,y,z) to
be computed on the fly (e.g. as a function of other stress
components).  E.g. we've done this for the thermostat
target temp in fix langevin.  It would be trickier for
pressures b/c they fluctuate so much.

Steve



On Fri, Apr 27, 2018 at 3:53 PM, Zhi Li <zhilisteve@...24...> wrote:
Hi All,

I'd like to deform a orthogonal simulation box in a way that the stress ratios between the three principle directions remain constant during the whole loading process. That is  if the stress is Px, Py, Pz in x,y,z direction, I want to keep Py/Px and Pz/Px constant. It seems that "fix deform" command will not do the trick since it could only control the strain rate rather than stress increment. I'm wondering if there is such command in lammps for this kind of loading condition? Any suggestions will be greatly appreciated. Thank you.

Best regards,

Zhi

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