qeq/reax and static charges in non-ReaxFF subsystem

Dear colleagues,

Let us consider the system consisting of two parts complying one to
ReaxFF ('ReaxFF subsystem') and and the other to some non-reactive force
field ('non-ReaxFF subsystem'). We achieve this, say, with command
pair_style hybrid reax/c controlfile lj/cut/coul/long 10.0

Atomic charges in ReaxFF subsystem must be equilibrated with fix
qeq/reax, while charges in non-ReaxFF subsystem are immutable. 'The QEq
method minimizes the electrostatic energy of the system by adjusting
the partial charge on individual atoms based on interactions with their
neighbors.' (from http://lammps.sandia.gov/doc/fix_qeq_reax.html)
The question is:
does qeq/reax algorithm takes into account static charges of
neighbor atoms from non-ReaxFF subsystem during minimization of
electrostatic energy via adjusting atomic charges within ReaxFF
subsystem?

Best regards,
Vadim

Dear colleagues,

Let us consider the system consisting of two parts complying one to
ReaxFF ('ReaxFF subsystem') and and the other to some non-reactive force
field ('non-ReaxFF subsystem'). We achieve this, say, with command
pair_style hybrid reax/c controlfile lj/cut/coul/long 10.0

Atomic charges in ReaxFF subsystem must be equilibrated with fix
qeq/reax, while charges in non-ReaxFF subsystem are immutable. 'The QEq
method minimizes the electrostatic energy of the system by adjusting
the partial charge on individual atoms based on interactions with their
neighbors.' (from LAMMPS Molecular Dynamics Simulator)
The question is:
does qeq/reax algorithm takes into account static charges of
neighbor atoms from non-ReaxFF subsystem during minimization of
electrostatic energy via adjusting atomic charges within ReaxFF
subsystem?

​you are worrying about the wrong problem. this is a bogus model. using
pair style hybrid causes more problems beyond charge equilibration.
1) how are you going to handle the "mixed" interactions be​tween the reaxff
part and the class 1 force field part?
you cannot use the lj/cut lennard-jones parameters, since they are not
consistent with the reaxff charges.
2) also, how are you going to handle the long-range coulomb? kspace
interactions are not computed for pairs of atoms, but considers the
(long-range) coulomb interactions of each atom with *all* atoms in the
system and their (infinite) periodic replica. reaxff instead uses a
wolf-sum approximation to long-range coulomb, if i remember correctly.

​to have a more consistent treatment of the interactions, you have to
follow an approach similar to what is done in QM/MM calculations. at the
very least you should implement an ONIOM-style mechanical coupling:
- you do a simulation of the *entire* system with lj/cut/coul/long and
kspace
- you do a simulation of the reaxff subsystem without the embedding
environment
- you do a simulation of the exact same subsystem​ but with lj/cut/coul/cut

now your total hamiltonian would be: U_total = U_all - U_lj/cut/coul/cut +
U_reaxff

​this will still have some inconsistencies (e.g. the embedding environment
cannot polarize the reaxff subsystem - hence the term "mechanical
coupling").
if you want to handle than, you'd have to add an electrostatic embedding,
similar to QM/MM couplings, and even that is not fully consistent, since
your static system is non-polarizable, while the reaxff subsystem is, and
thus there is an non-physical unbalanced energy transfer during MD.

​axel.​