newton command — LAMMPS documentation

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# newton command

## Syntax

    newton flag
    newton flag1 flag2

- flag = on or off for both pairwise and bonded interactions

- flag1 = on or off for pairwise interactions

- flag2 = on or off for bonded interactions

## Examples

    newton off
    newton on off

## Description

This command turns Newton’s third law on or off for pairwise and bonded interactions. For most problems, setting Newton’s third law to on means a modest savings in computation at the cost of two times more communication. Whether this is faster depends on problem size, force cutoff lengths, a machine’s compute/communication ratio, and how many processors are being used.

Setting the pairwise newton flag to off means that if two interacting atoms are on different processors, both processors compute their interaction and the resulting force information is not communicated. Similarly, for bonded interactions, newton off means that if a bond, angle, dihedral, or improper interaction contains atoms on 2 or more processors, the interaction is computed by each processor.

LAMMPS should produce the same answers for any newton flag settings, except for round-off issues.

With run_style respa and only bonded interactions (bond, angle, etc) computed in the innermost timestep, it may be faster to turn newton off for bonded interactions, to avoid extra communication in the innermost loop.

## Restrictions

The newton bond setting cannot be changed after the simulation box is defined by a read_data or create_box command.

## Related commands

run_style respa

## Default

    newton on