mulliken charge

My system has mulliken charge.can i write zero charge for atoms in
data file and use fix qeq/reax command for a non-ReaxFF potential
(lj/cut/coul/ tip4p/long) . Is that right?

No, currently fix qeq/reax may only be used with reax/c. I think Ray had plans to implement the universal qeq fix, but as far as I know it isn't ready yet.

Oleg

29.11.2013, 20:02, "meymanat zokaie" <[email protected]>:

Correct me if I am wrong but from reading the manual quickly I believe the qeq/reax fix can but used with other potential (no restriction shows on the page) as opposed to the fix qeq/comb whihc is only supported with the COMB pairstyle. I guess the potential problem with mixing the fix with other styles are exclusions to appear in the neighbor list as a result of bonded interactions.
Again, I never tried using this combination thus not 100% sure my quick answer is bulletproof.
Carlos

Maybe it is not clear on the doc page, but we have disabled the use of
fix qeq/reax with non-reaxff potentials. This is because it is not
reasonable to assign charges on atoms other than the charge values
that these fixed-charge potentials were fitted and parameterized with.
For example, TIP4P water models have its own charge values (please
see http://lammps.sandia.gov/doc/Section_howto.html#howto_8) that
should not be changed.

Ray

I guess that penalizes users who might be interested in using lammps as a library to create their own new potential parametrizations for systems where the charge equilibration process is described via QEq. Personally, I would not have tried to restrain people from being creative by limiting the code list of features. Ultimately, if the user idea is innovative or plain stupidity that goes much beyond the lammps developers responsibility. In any case, you guys are in charge and know better what your overall mission is.
Carlos

Carlos,

That is a good point. Maybe it is a good idea to allow the use of fix
qeq/reax with non-reax potentials but with a warning message such as
the "I hope you know what you are doing" message in VASP. :slight_smile:

But for those who wish to use fix qeq/reax with non-reax potentials,
they can comment out lines 319 and 320 of fix_qeq_reax.cpp, and this
will allow fix qeq/reax with any pair_styles.

Ray

Good fish steaks come with bones too :wink: Don’t take away the fish from the menu please, just warn me about the bones…the VASP approach feels sufficient to me.

Carlos

My system has mulliken charge.can i write zero charge for atoms in

what do you mean by "my system has mulliken charge"?

data file and use fix qeq/reax command for a non-ReaxFF potential
(lj/cut/coul/ tip4p/long) . Is that right?

that is likely to be a bad idea. charge equilibration is a procedure
that has to be well understood to use it correctly or you risk getting
bad results. if you want to model TIP4P water, then it is definitely
not a good idea. the partial charges for tip4p are well defined.

axel.