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.