Charge Distribution in Reax and COMB

Hello LAMMPS users,

I am modeling silica (5184 atoms) under an NPT ensemble at 300K and 1 bar with ReaxFF and COMB. My timestep for both simulations is 0.2fs.

I have read an article in MRS Bulletin stating that ReaxFF assigns charges of +1.346 and -0.673 to silicon and oxygen atoms respectively.

I have taken a script from the example/comb input file for HfO2 to calculate the reduced charge q1 and q2 and I then write these values to a file.

After long simulation runs (several ps), for ReaxFF I see that the charge fluctuates around 1.6 and -0.8 for silicon and oxygen respectively, instead of 1.346 and -0.673 as reported.

Shouldn’t I be seeing the charges that ReaxFF is said to “assign?”

Additionally, for COMB, reported charges are 2.92 and -1.46, and I am seeing 2.91 and -1.453 (so COMB is closer to reported values leading me to believe there is nothing wrong with my geometry or the way in which I set up the simulation).

Has anyone modeled silica with these potentials and reproduced the charges reported?

Best Regards,


By “assign” it meant that the equilibrium charges are calculated from QEq. You probably did not use the same ReaxFF force field that is used in the said publication.


The publication that I am referring to, as I am sure you know, you were a co-author on. I am using one of the ReaxFF force fields provided to me by Dr. Adri van Duin.

The title of the file is: Reactive MD-force field: Si/SiO/SiN interactions with water and Na+ Fogarty et al.

I understand that my problem does not deal with water. Is there another ReaxFF force field parameter set for Si/SiO2 interface only?

By the way, I appreciate the MRS Bulletin paper: It is highly informative of COMB and ReaxFF.

More specifically for COMB10, the charges I am seeing are say, 2.905 instead of 2.92. Is this within the realm that would be considered accurate? I speculate that it is, but I do not want to mess up further simulations before getting the basics down.



I believe there are many other ReaxFF descriptions for Si/SiO2, and I don’t have them. You will have to look at the Literature or ask Adri.

The small difference in charge is negligible.


Ok thank you for the quick response. Email to Adri has been sent.