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This package contains 2 fix commands, "fix qtb" and
"fix qbmsst" which involve quantum nuclear effects.
What is quantum nuclear effects (in molecular dynamics
simulation)?
Quantum treatment of the vibrational modes will
introduce zero point energy into the system, alter
the energy power spectrum and bias the heat capacity
from the classical limit. classical MD leaves out
these effects completely and thus need to be corrected.
When should I consider quantum nuclear effects in
my simulation?
(1) When you want to model systems at temperatures
lower than their classical limits. This is especially
important for materials with a large population of
hydrogen atoms and thus higher classical limits.
(2) In MD simulations when temperatures ramp up
across (or starting from somewhere close to) the
classical limits, tiny differences between the
classical and quantum heat capacity can accumulate
and cause the final state to deviate (could be as
large as 30 %).
What command should I use regarding these two cases?
(1) "fix qtb" provides quantum nulcear correction
through a colored thermostat and can be used with
other time integration schemes like fix nve or
fix nph. In this case, you tell "fix qtb" the temperature
and the other time integration scheme either the
pressure ("fix nph") or the volume ("fix nve").
(2) "fix qbmsst" deals with shock MD simulations
in which cases the temperature may increase a lot.
It enables quantum nuclear correction of a multi-scale
shock technique simulation by coupling the quantum
thermal bath with the shocked system. It is an independent
command from "fix msst" and does not assume that
the SHOCK package has been installed.
See the doc page for the fix qtb and fix qbmsst
command for detailed usage instructions.
Where can I find examples of these two commands?
There are example scripts for using this package in
examples/USER/qtb, including one "fix qtb" and one
"fix qbmsst" example for each of alpha quartz and
methane. Running the alpha quartz example requires
installation of the kspace package while the methane
example requires the REAX package for the force field.
Authors Information and Contact
The person who created this package is Yuan Shen
(sy0302 at stanford.edu) at Stanford University.
Contact him directly if you have questions.
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