how to specify precision in the fix qeq/comb command when use comb3 potential

Dear all,

I’m using comb3 potential to simulate pure Al. It requires the fix qeq/comb command specified in the input script, the syntax of the fix qeq/comb3 is as follows:

fix ID group-ID qeq/comb dtq precision keyword value …

Here is a description of the ‘precision’ paramter in this command: precision = the convergence and threshold criterion of the maximum charge force. The charge force of asystem scales with the temperature of the system. The typical charge force of a system at 300 K is about 0.2 V. In this case, it is a waste of time to equilibrate the charges such that the maximum charge force is way smaller than 0.2 V. The recommended {precision} value is 1.0e-3 for below 50 K, 0.1 for room temperature and 0.5 for 1000 K or above.

I don’t understand the above sentences completely. Does it mean when we specify ‘precision’, we only need to consider temperature? I’m simulating pure Al at temperature higher than 1000K, will it be ok to let ‘precision’ equal 0.5? If I let ‘precision’ equal 0.5, I find from the dump file that charges of each atom are not zero, how to explain this since Al is electrical neutrality?

Best wishes,


What you are using (fix qeq/comb3) is not part of the LAMMPS official distribution so you should ask the person who made the custom changes.


Hi Ray,

Thanks for your reply. I’ll contact the developer.

Best wishes,