Charge Equilibration During minimization

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1

Hi,
If I try to minimize a system with a charge dependent potential (such as COMB) using
minimize command, during the minimization steps fix qeq will not invoked. As a result of this
during the minimization charges will be constant. It is quite unphysical. Is there anyway to do charge active
minimization?
dundar.

2

first off,

please never *ever* reply to a digested e-mail without removing the
irrelevant content. you'll just flood people's inboxes with useless
content for no good reason. there is enough spam floating around
already.
since you are not replying to any of the e-mails, please don't use
reply in the first place, but post a new e-mail. it will make life
easier for people that receive a lot of e-mail, since then your post
will be grouped correctly by default and without requiring manual
effort.

thanks for your consideration.

Hi,
If I try to minimize a system with a charge dependent potential (such as COMB) using
minimize command, during the minimization steps fix qeq will not invoked. As a result of this
during the minimization charges will be constant. It is quite unphysical. Is there anyway to do charge active
minimization?

you could just do a damped dynamics. that is assign a reasonably high
temperature and do an MD with fix viscous and run until your kinetic
energy is small enough. like with all minimizations you have to check
for reaching a proper minimum.

axel.

3

That's a good question for Ray.

Steve

4

Thanks for the quick reply.
I am little confused:
If I use
fix viscous
and
fix npt together will "fix viscous" still damp the velocities of atoms? I assume "fix npt" will try to keep the
temperature at given level by adding/scaling velocities, so fix viscous should not work with "fix npt".
If I use "fix nve" it may work however in this case box will remain constant. What I am really trying to do is

fix box/relax aniso
minimize 1e-5 1e-6 500 5000

command with charged version.

dundar.

5

Thanks for the quick reply.
I am little confused:
If I use
fix viscous
and
fix npt together will "fix viscous" still damp the velocities of atoms? I assume "fix npt" will try to keep the
temperature at given level by adding/scaling velocities, so fix viscous should not work with "fix npt".
If I use "fix nve" it may work however in this case box will remain constant. What I am really trying to do is

you can use fix nph instead of fix npt that will allow you to remove
kinetic energy from the system.

axel.

6

This is a different Q than you first asked. Using fix viscous with
any thermostat, like fix npt is a bad idea. But LAMMPS won't stop
you from doing bad things. YOu could try fix nph which doesn't
have a thermostat. Or you could wait for Ray to answer your original Q.

Steve

7

I have just updated fix qeq/comb to allow charge equilibration during
minimization. Please find the attached routines.

Ray

fix_qeq_comb.cpp (8.11 KB)

fix_qeq_comb.h (1.96 KB)

8

See the Feb 22 patch.

Steve