$$\renewcommand{\AA}{\text{Å}}$$

# 3.5. Pair styles¶

Classes that compute pairwise non-bonded interactions are derived from the Pair class. In LAMMPS, pairwise calculation include many-body potentials such as EAM, Tersoff, or ReaxFF where particles interact without an explicit bond topology but include interactions beyond pairwise non-bonded contributions. New styles can be created to add support for additional pair potentials to LAMMPS. When the modifications are small, sometimes it is more effective to derive from an existing pair style class. This latter approach is also used by Accelerator packages where the accelerated style names differ from their base classes by an appended suffix.

The file src/pair_lj_cut.cpp is an example of a Pair class with a very simple potential function. It includes several optional methods to enable its use with run_style respa and compute group/group.

Here is a brief list of some the class methods in the Pair class that must be or may be overridden in a derived class.

Required

“pure” methods that must be overridden in a derived class

compute

workhorse routine that computes pairwise interactions

settings

processes the arguments to the pair_style command

coeff

set coefficients for one i,j type pair, called from pair_coeff

Optional

methods that have a default or dummy implementation

init_one

perform initialization for one i,j type pair

init_style

style initialization: request neighbor list(s), error checks

init_list

Neighbor class callback function to pass neighbor list to pair style

single

force/r and energy of a single pairwise interaction between 2 atoms

compute_inner/middle/outer

versions of compute used by rRESPA

memory_usage

return estimated amount of memory used by the pair style

modify_params

process arguments to pair_modify command

extract

provide access to internal scalar or per-type data like cutoffs

extract_peratom

provide access to internal per-atom data

setup

initialization at the beginning of a run

finish

called at the end of a run, e.g. to print

write & read_restart

write/read i,j pair coeffs to restart files

write & read_restart_settings

write/read global settings to restart files

write_data

write Pair Coeffs section to data file

write_data_all

write PairIJ Coeffs section to data file

pack & unpack_forward_comm

copy data to and from buffer if style uses forward communication

pack & unpack_reverse_comm

copy data to and from buffer if style uses reverse communication

reinit

reset all type-based parameters, called by fix adapt for example

reset_dt

called when the time step is changed by timestep or fix reset/dt

Here is a list of flags or settings that should be set in the constructor of the derived pair class when they differ from the default setting.

Name of flag

Description

default

single_enable

1 if single() method is implemented, 0 if missing

1

respa_enable

1 if pair style has compute_inner/middle/outer()

0

restartinfo

1 if pair style writes its settings to a restart

1

one_coeff

1 if only a pair_coeff * * command is allowed

0

manybody_flag

1 if pair style is a manybody potential

0

unit_convert_flag

value != 0 indicates support for unit conversion

0

no_virial_fdotr_compute

1 if pair style does not call virial_fdotr_compute()

0

writedata

1 if write_data() and write_data_all() are implemented

0

comm_forward

size of buffer (in doubles) for forward communication

0

comm_reverse

size of buffer (in doubles) for reverse communication

0

ghostneigh

1 if cutghost is set and style uses neighbors of ghosts

0

finitecutflag

1 if cutoff depends on diameter of atoms

0

reinitflag

1 if style has reinit() and is compatible with fix adapt

0

ewaldflag

1 if compatible with kspace_style ewald

0

pppmflag

1 if compatible with kspace_style pppm

0

msmflag

1 if compatible with kspace_style msm

0

dispersionflag

1 if compatible with ewald/disp or pppm/disp

0

tip4pflag

1 if compatible with kspace_style pppm/tip4p

0

dipoleflag

1 if compatible with dipole kspace_style

0

spinflag

1 if compatible with spin kspace_style

0