fix qeq/comb

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


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.


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.