lammps/doc/pair_srp.txt

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pair_style srp command :h3

[Syntax:]

pair_style srp cutoff bond_type dist keyword value ...

cutoff = global cutoff for SRP interactions (distance units) :ulb,l
bond_type = bond type to apply SRP interactions :l
dist = {min} or {mid} :l
zero or more keyword/value pairs may be appended :l
keyword = {exclude} :l
  {exclude} value = {yes} or {no} :pre
:ule

[Examples:]

pair_style hybrid dpd 1.0 1.0 12345 srp 0.8 1 mid exclude yes
pair_coeff 1 1 dpd 60.0 4.5 1.0 
pair_coeff 1 2 none 
pair_coeff 2 2 srp 100.0 0.8 :pre

pair_style hybrid srp 0.8 1 mid exclude yes
pair_coeff 1 1 none 
pair_coeff 1 2 none 
pair_coeff 2 2 srp 100.0 0.8 :pre 

[Description:]

Style {srp} computes a soft segmental repulsive potential (SRP) that
acts between pairs of bonds. This potential is useful for preventing
bonds from passing through one another when a soft non-bonded
potential acts between beads in, for example, DPD polymer chains.  An
example input script that uses this command is provided in
examples/USER/srp.

Bonds of type {btype} interact with one another through a
bond-pairwise potential, such that the force on bond {i} due to bond
{j} is as follows

:c,image(Eqs/pair_srp1.jpg)

where {r} and {rij} are the distance and unit vector between the two
bonds. The {mid} option computes {r} and {rij} from the midpoint
distance between bonds. The {min} option computes {r} and {rij} from
the minimum distance between bonds. The force acting on a bond is
mapped onto the two bond atoms according to the lever rule,

:c,image(Eqs/pair_srp2.jpg)

where {L} is the normalized distance from the atom to the point of
closest approach of bond {i} and {j}. The {mid} option takes {L} as
0.5 for each interaction as described in "(Sirk)"_#Sirk.

The following coefficients must be defined via the
"pair_coeff"_pair_coeff.html command as in the examples above, or in
the data file or restart file read by the "read_data"_read_data.html
or "read_restart"_read_restart.html commands:

{C} (force units)
{Rc} (distance units) :ul

The last coefficient is optional. If not specified, the global cutoff
is used.

IMPORTANT NOTE: Pair style srp considers each bond of type {btype} as
a fictitious particle of type {bptype}, where {bptype} is the largest
atom type in the system.  These "bond particles" are inserted at the
beginning of the run, and serve as placeholders that define the
position of the bonds.  This allows neighbor lists to be constructed
and pairwise interactions to be computed in almost the same way as is
done for point particles.  Because bonds interact only with other
bonds, "pair_style hybrid"_pair_hybrid.html should be used to turn off
interactions between atom type {bptype} and all other types of atoms.
An error will be flagged if "pair_style hybrid"_pair_hybrid.html is
not used.  Further, only bond particles should be given an atom type
of {bptype}; a check is done at the beginning of the run to ensure
there are no regular atoms of {bptype}.

The optional {exclude} keyword determines if forces are computed
between first neighbor (directly connected) bonds.  For a setting of
{yes} they are, for {no} they are not.

Pair style {srp} turns off normalization of thermodynamic properties
by particle number, as if the command "thermo_modify norm
no"_thermo_modify.html had been issued.  A warning will be given if
this setting is overridden in the input script.

The pairwise energy associated with style {srp} is shifted to be zero
at the cutoff distance {Rc}.

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

This pair styles does not support mixing.  

This pair style does not support the "pair_modify"_pair_modify.html
shift option for the energy of the pair interaction. Note that as
discussed above, the energy term is already shifted to be 0.0 at the
cutoff distance {rc}.

The "pair_modify"_pair_modify.html table option is not relevant for
this pair style.

This pair style does not support the "pair_modify"_pair_modify.html
tail option for adding long-range tail corrections to energy and
pressure.

This pair style writes global and per-atom information to "binary
restart files"_restart.html. Pair srp should be used with "pair_style
hybrid"_pair_hybrid.html, thus the pair_coeff commands need to be
specified in the input script when reading a restart file.

This pair style can only be used via the {pair} keyword of the
"run_style respa"_run_style.html command.  It does not support the
{inner}, {middle}, {outer} keywords.

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[Restrictions:]

This pair style is part of the USER-MISC package. It is only enabled
if LAMMPS was built with that package. See the Making LAMMPS section
for more info.

This pair style must be used with "pair_style hybrid"_pair_hybrid.html.

This pair style requires the "newton"_newton.html command to be {on}
for non-bonded interactions.

[Related commands:]

"pair_style hybrid"_pair_hybrid.html, "pair_coeff"_pair_coeff.html,
"pair dpd"_pair_dpd.html

[Default:]

The default keyword value is exclude = yes.

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:link(Sirk)
[(Sirk)] Sirk TW, Slizoberg YR, Brennan JK, Lisal M, Andzelm JW, J
Chem Phys, 136 (13) 134903, 2012.