lammps/doc/pair_hbond_dreiding.txt

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pair_style hbond/dreiding/lj command :h3
pair_style hbond/dreiding/morse command :h3

[Syntax:]

pair_style style distance_cutoff angle_cutof N :pre

style = {hbond/dreiding/lj} or {hbond/dreiding/morse} 
n = cosine angle periodicity
distance_cutoff = global cutoff for Donor-Acceptor interactions (distance units)
angle_cutoff = global angle cutoff for Acceptor-Hydrogen-Donor
interactions (degrees) :ul

[Examples:]

pair_style hbond/dreiding/lj 4 5.0 90
pair_coeff * * 3 i 100.0 3.1
pair_coeff * * 2*5 i 100.0 3.1 2 200.0 :pre

pair_style hbond/dreiding/morse 2 4.6 75.0 
pair_coeff * * 3 j 100.0 1.0 2.0
pair_coeff * * 2*5 j 100.0 1.0 2.0 4 :pre

[Description:]

The {hbond/dreiding} styles compute the Acceptor-Hydrogen-Donor (AHD)
3-body hydrogen bond interaction for the
"DREIDING"_Section_howto.html#4_4 force field, given by:

:c,image(Eqs/pair_hbond_dreiding.jpg)

where Rc is the distance cutoff, theta_c is the angle cutoff, and n is
the cosine periodicity.

Here, {r} is the radial distance between the donor (D) and acceptor
(A) atoms and {theta} is the bond angle between the acceptor, the
hydrogen (H) and the donor atoms:

:c,image(Eqs/dreiding_hbond.jpg)

These 3-body interactions can be defined for pairs of acceptor and
donor atoms, based on atom types.  For each donor/acceptor atom pair,
the 3rd atom in the interaction is a hydrogen permanently bonded to
the donor atom, e.g. in a bond list read in from a data file via the
"read_data"_read_data.html command.  The atom types of possible
hydrogen atoms for each donor/acceptor type pair are specified by the
"pair_coeff"_pair_coeff.html command (see below).

Style {hbond/dreiding/lj} is the original DREIDING potential of
"(Mayo)"_#Mayo.  It uses a LJ 12/10 functional for the Donor-Acceptor
interactions. To match the results in the original paper, use n = 4.

Style {hbond/dreiding/morse} is an improved version using a Morse
potential for the Donor-Acceptor interactions. "(Liu)"_#Liu showed
that the Morse form gives improved results for Dendrimer simulations,
when n = 2.

See this "howto section"_Section_howto.html#4_4 of the manual for more
information on the DREIDING forcefield.

Because the Dreiding hydrogen bond potential is only one portion of
an overall force field which typically includes other pairwise
interactions, it is common to use it as a sub-style in a "pair_style
hybrid or hybrid/overlay"_pair_hybrid.html command.

The following coefficients must be defined for pairs of eligible
donor/acceptor types via the "pair_coeff"_pair_coeff.html command as
in the examples above.

IMPORTANT NOTE: Unlike other pair styles and their associated
"pair_coeff"_pair_coeff.html commands, you do not need to specify
pair_coeff settings for all possible I,J type pairs.  Only I,J type
pairs for atoms which act as joint donors/acceptors need to be
specified; all other type pairs are assumed to be inactive.
 
IMPORTANT NOTE: A "pair_coeff"_pair_coeff.html command can be
speficied multiple times for the same donor/acceptor type pair.  This
enables multiple hydrogen types to be assigned to the same
donor/acceptor type pair.  For other pair_styles, if the pair_coeff
command is re-used for the same I.J type pair, the settings for that
type pair are overwritten.  For the hydrogen bond potentials this is
not the case; the settings are cummulative.  This means the only way
to turn off a previous setting, is to re-use the pair_style command
and start over.

For the {hbond/dreiding/lj} style the list of coefficients is as
follows:

K = hydrogen atom type = 1 to Ntypes
donor flag = {i} or {j}
epsilon (energy units)
sigma (distance units)
n = exponent in formula above
distance cutoff (distance units)
angle cutoff (degrees) :ul

For the {hbond/dreiding/morse} style the list of coefficients is as
follows:

K = hydrogen atom type = 1 to Ntypes
donor flag = {i} or {j}
D0 (energy units)
alpha (1/distance units)
r0 (distance units)
n = exponent in formula above
distance cutoff (distance units)
angle cutoff (degrees) :ul

A single hydrogen atom type K can be specified, or a wild-card
asterisk can be used in place of or in conjunction with the K
arguments to select multiple types as hydrogens.  This takes the form
"*" or "*n" or "n*" or "m*n".  See the "pair_coeff"_pair_coeff command
doc page for details.

If the donor flag is {i}, then the atom of type I in the pair_coeff
command is treated as the donor, and J is the acceptor.  If the donor
flag is {j}, then the atom of type J in the pair_coeff command is
treated as the donor and I is the donor.  This option is required
because the "pair_coeff"_pair_coeff.html command requires that I <= J.

Epsilon and sigma are settings for the hydrogen bond potential based
on a Lennard-Jones functional form.  Note that sigma is defined as the
zero-crossing distance for the potential, not as the energy minimum at
2^(1/6) sigma.

D0 and alpha and r0 are settings for the hydrogen bond potential based
on a Morse functional form.

The last 3 coefficients for both styles are optional.  If not
specified, the global n, distance cutoff, and angle cutoff specified
in the pair_style command are used.  If you wish to only override the
2nd or 3rd optional parameter, you must also specify the preceding
optional parameters.

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[Mixing, shift, table, tail correction, restart, rRESPA info]:

These pair styles do not support mixing. You must explicitly identify
each donor/acceptor type pair.

These styles do not support the "pair_modify"_pair_modify.html shift
option for the energy of the interactions.

The "pair_modify"_pair_modify.html table option is not relevant for
these pair styles.

These pair styles do not support the "pair_modify"_pair_modify.html
tail option for adding long-range tail corrections to energy and
pressure.

These pair styles do not write their information to "binary restart
files"_restart.html, so pair_style and pair_coeff commands need to be
re-specified in an input script that reads a restart file.

These pair styles can only be used via the {pair} keyword of the
"run_style respa"_run_style.html command.  They do not support the
{inner}, {middle}, {outer} keywords.

These pair style tally a count of how many hydrogen bonding
interactions they calculate each timestep.  This quantity can be
accessed via the "compute pair"_compute_pair.html command as a vector
of values of length 1.

To print this quantity to the log file (with a descriptive column
heading) the following commands could be included in an input script:

compute hb all pair hbond/dreiding/lj
variable hb equal c_hb\[1\]
thermo_style custom step temp epair v_hb :pre

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[Restrictions:] none

[Related commands:]

"pair_coeff"_pair_coeff.html

[Default:] none

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:link(Mayo)
[(Mayo)] Mayo, Olfason, Goddard III, J Phys Chem, 94, 8897-8909
(1990).

:link(Liu)
[(Liu)] Liu, Bryantsev, Diallo, Goddard III, J. Am. Chem. Soc 131 (8)
2798 (2009)