fix qeq/comb

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1

Fix qeq/comb uses the extended Lagrangian method. LAMMPS distro includes both comb2 and comb3.

Ray

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Thanks for the response Ray.

I’m confused by the Nevery argument for fix qeq/comb. If this is an extended Lagrangian method, shouldn’t the dynamics for the charge be propagated every time step, or even using a RESPA style multiple timestep method?

Also, I’m assuming the charge dynamics are damped, but there is no argument for controlling the damping parameter. I would like to test what happens without damping the dynamics. Is it possible to control the damping parameter with fix qeq/comb?

I may just need to dig into the source code to get a better understanding how this works.

Susan told me to download a patch from her groups website, rather than using the fix packaged with LAMMPS for charge equilibration. However, we are trying to understand what the two versions do differently.

Seth

3

The Nevery option just provides an option to reduce the burden of variable charge equilibration. For a well charge-equilibrated system it usually makes little difference if qeq is called every or every 2-5 steps.

Fix qeq/comb is quite rudimentary and was developed/coded only for COMB2/COMB3. If you are looking for a more advanced extended Lagrangian qeq method, please check out fix qeq/dynamic and fix qeq/fire.

Unreleased fix qeq/comb from Susan’s website contains additional options for qeq/comb – somewhat similar to fix/dynamic but focuses on COMB2/3 as well.

Ray