This directory has two input scripts that illustrates how to use fix
charge_regulation in LAMMPS to perform coarse-grained molecular dynamics
(MD) simulations with incorporation of charge regulation effects. The
charge regulation is implemented via Monte Carlo (MC) sampling following
the reaction ensemble MC approach, producing a MC/MD hybrid tool for
modeling charge regulation in solvated systems.

The script `in.chreg-acid` sets up a simple weak acid electrolyte
(pH=7,pKa=6,pI=3). Four different types of MC moves are implemented:
acid protonation & de-protonation, and monovalent ion pair insertion and
deletion. Note here we have grouped all free monovalent ions into a
single type, a physically natural choice on the level of coarse-grained
primitive electrolyte models, which increases the calculation
performance but has no effects on thermodynamic observables. The
variables such as pH, pKa, pI, and lb at the top of the input script can
be adjusted to play with various simulation parameters. The cumulative
MC attempted moves and cumulative number of accepted moves, as well as,
current number of neutral and charged acid particles, neutral and
charged base particles (in this example always 0), and the current
number of free cations and anions in the system are printed in the
output.

The script `in.chreg-polymer` sets up a weak poly-electrolyte chain of
N=80 beads. Each bead is a weak acid with pKa=5 and solution has pH=7
and monovalent salt chemical potential pI=3. In this example, we choose
to treat salt ions, protons, and hydroxyl ions separately, which results
in 5 types of MC moves: acid [type 1] protonation & de-protonation (with
protons [type 4] insertion & deletion), acid [type 1] protonation &
de-protonation (with salt cation [type 2] insertion & deletion), water
self-ionization (insertion and deletion of proton [type4] and hydroxyl
ion [type 5] pair), insertion and deletion of monovalent salt pair [type
2 and type 3] , insertion and deletion of a proton [type4] and salt
anion [type 3].  The current number of neutral and charged acid
particles, the current number of free salt cations and anions, and the
current number of protons and hydroxyl ions are printed in the output.

