# compute bond/local command

## Syntax

```
compute ID group-ID bond/local value1 value2 ...
```

ID, group-ID are documented in

*compute*commandbond/local = style name of this compute command

one or more values may be appended

value =

*dist*or*engpot*or*force*or*engvib*or*engrot*or*engtrans*or*omega*or*velvib**dist*= bond distance*engpot*= bond potential energy*force*= bond force*engvib*= bond kinetic energy of vibration*engrot*= bond kinetic energy of rotation*engtrans*= bond kinetic energy of translation*omega*= magnitude of bond angular velocity*velvib*= vibrational velocity along the bond length

## Examples

```
compute 1 all bond/local engpot
compute 1 all bond/local dist engpot force
```

## Description

Define a computation that calculates properties of individual bond interactions. The number of datums generated, aggregated across all processors, equals the number of bonds in the system, modified by the group parameter as explained below.

The value *dist* is the current length of the bond.

The value *engpot* is the potential energy for the bond,
based on the current separation of the pair of atoms in the bond.

The value *force* is the magnitude of the force acting between the
pair of atoms in the bond.

The remaining properties are all computed for motion of the two atoms relative to the center of mass (COM) velocity of the 2 atoms in the bond.

The value *engvib* is the vibrational kinetic energy of the two atoms
in the bond, which is simply 1/2 m1 v1^2 + 1/2 m2 v2^2, where v1 and
v2 are the magnitude of the velocity of the 2 atoms along the bond
direction, after the COM velocity has been subtracted from each.

The value *engrot* is the rotationsl kinetic energy of the two atoms
in the bond, which is simply 1/2 m1 v1^2 + 1/2 m2 v2^2, where v1 and
v2 are the magnitude of the velocity of the 2 atoms perpendicular to
the bond direction, after the COM velocity has been subtracted from
each.

The value *engtrans* is the translational kinetic energy associated
with the motion of the COM of the system itself, namely 1/2 (m1+m2)
Vcm^2 where Vcm = magnitude of the velocity of the COM.

Note that these 3 kinetic energy terms are simply a partitioning of the summed kinetic energy of the 2 atoms themselves. I.e. total KE = 1/2 m1 v1^2 + 1/2 m2 v2^2 = engvib + engrot + engtrans, where v1,v2 are the magnitude of the velocities of the 2 atoms, without any adjustment for the COM velocity.

The value *omega* is the magnitude of the angular velocity of the
two atoms around their COM position.

The value *velvib* is the magnitude of the relative velocity of the
two atoms in the bond towards each other. A negative value means the
2 atoms are moving toward each other; a positive value means they are
moving apart.

Note that all these properties are computed for the pair of atoms in a bond, whether the 2 atoms represent a simple diatomic molecule, or are part of some larger molecule.

The local data stored by this command is generated by looping over all
the atoms owned on a processor and their bonds. A bond will only be
included if both atoms in the bond are in the specified compute group.
Any bonds that have been broken (see the *bond_style*
command) by setting their bond type to 0 are not included. Bonds that
have been turned off (see the *fix shake* or
*delete_bonds* commands) by setting their bond type
negative are written into the file, but their energy will be 0.0.

Note that as atoms migrate from processor to processor, there will be
no consistent ordering of the entries within the local vector or array
from one timestep to the next. The only consistency that is
guaranteed is that the ordering on a particular timestep will be the
same for local vectors or arrays generated by other compute commands.
For example, bond output from the *compute property/local* command can be combined
with data from this command and output by the *dump local*
command in a consistent way.

Here is an example of how to do this:

compute 1 all property/local btype batom1 batom2 compute 2 all bond/local dist engpot dump 1 all local 1000 tmp.dump index c_1[*] c_2[*]

**Output info:**

This compute calculates a local vector or local array depending on the number of keywords. The length of the vector or number of rows in the array is the number of bonds. If a single keyword is specified, a local vector is produced. If two or more keywords are specified, a local array is produced where the number of columns = the number of keywords. The vector or array can be accessed by any command that uses local values from a compute as input. See this section for an overview of LAMMPS output options.

The output for *dist* will be in distance *units*. The
output for *velvib* will be in velocity *units*. The output
for *omega* will be in velocity/distance *units*. The
output for *engtrans*, *engvib*, *engrot*, and *engpot* will be in
energy *units*. The output for *force* will be in force
*units*.

## Restrictions

none