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newton.txt
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.. index:: newton
newton command
==============
Syntax
""""""
.. parsed-literal::
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
""""""""
.. parsed-literal::
newton off
newton on off
Description
"""""""""""
This command turns Newton's 3rd law *on* or *off* for pairwise and
bonded interactions. For most problems, setting Newton's 3rd 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 :doc:`run_style <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 :doc:`read_data <read_data>` or
:doc:`create_box <create_box>` command.
Related commands
""""""""""""""""
:doc:`run_style <run_style>` respa
Default
"""""""
.. parsed-literal::
newton on
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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