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Added comment about momentum under NPT 

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14397 f3b2605a-c512-4ea7-a41b-209d697bcdaa
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athomps 2015-12-23 01:40:29 +00:00
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doc/fix_nh.txt
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@ -440,6 +440,16 @@ limiting factor for numerical stability. Both factorizations are
time-reversible and can be shown to preserve the phase space measure
of the underlying non-Hamiltonian equations of motion.
NOTE: This implementation has been shown to conserve linear momentum
up to machine precision under NVT dynamics. Under NPT dynamics,
for a system with zero initial total linear momentum, the total
momentum fluctuates close to zero. It may occasionally undergo brief
excursions to non-negligible values, before returning close to zero.
Over long simulations, this has the effect of causing the center-of-mass
to undergo a slow random walk. This can be mitigated by resetting
the momentum at infrequent intervals using the
"fix momentum"_fix_momentum.html command.
:line
The fix npt and fix nph commands can be used with rigid bodies or