fix qeq/ecomb command

Syntax:

fix ID group-ID qeq/comb dtq grcathode granode V keyword value ... 
ID, group-ID are documented in fix command
qeq/comb = style name of this fix command
dtq = time step of this fix command (attosecond unit)
precision = the convergence and threshold criterion of the maximum charge force
grcathode = group-ID for cathode
granode = group-ID for anode
V = applied potential between cathode and anode (unit V)
zero or more keyword/value pairs may be appended
keyword = precision or nevery or optimize or verlet or appq or file 
  nevery value = Nevery
    Nevery = perform this fix every Nevery time steps
  optimize args = none
  verlet args = none
  maxloop value = Maxloop
    Maxloop = maximum number of loops of the charge optimization
  appq value = Appq
    Appq = the initial net charge that is applied to the system (e unit)
  file value = filename and qthermo
    filename = name of file to write charge dynamics info to
    qthermo = write the charge dynamics info to file every qthermo time steps 
Examples:

fix 1 etrode qeq/ecomb 2.0 grcathode granode 3.0
fix 1 etrode qeq/ecomb 2.0 1e-2 grcathode granode 3.0 nevery 10 verlet

Description:

Offset the charge force between cathode and anode atoms by {V} due to an external 
electric voltage being applied to the system. The electrolyte are implicitly
included as (group_ID - grcathode - granode). However, as indicated in reference below, it is recommended to use this fix command on electrodes only and use separate fix qeq/comb command on eletrolyte.

Please note the difference
between this fix and "fix efield"_fix_efield.html. The "fix
efield"_fix_efield.html is adding qE force on each charged atom. Whereas this
fix is adding {V}/2 on the charge force of the cathode atom and deducting {V}/2 on
charge force of the anode. The electronegativity of an atom is defined as the derivative of
the energy over the charge of the atom. The charge force is defined as
difference between the mean value of the electronegativity of the system and the electronegativity of
the atom.

Though the idea of offset the charge forces can be applied to any variable atomic charge models, this fix is
currently limited to offset the charge forces obtained from "fix
qeq/comb"_fix_qeq_comb.html. In fact, this fix only adds three variables:
{grcathode}, {granode} and {V}. The rest variables in this fix is the same as "fix
qeq/comb"_fix_qeq_comb.html.

In order to conserve the electronegativity of each step of the charge
dynamics, the real offset value is
{V}*N_anode/(N_cathode+N_anode)*{V} for cathode atoms and
-{V}*N_cathode/(N_cathode+N_anode) for anode atoms, where N_cathode and
N_anode are the number of cathode and anode atoms.


[(ECOMB)]  T. Liang, A. C. Antony, S. A. Akhade, M. J. Janik and S. B.
Sinnott, in preparation.