[CPMD-list] nonadiabaticity with staging mode path integral

[Lan-Feng Yuan]

  Dear CPMD users,

  In simulating a single classical H2O molecule using CP-MD and a quantum one
using path integral MD, there is the issue of adiabaticity between electrons and
nuclei. Since H2O has a large gap, I do not use a thermostat for electrons. First
I do the classical (CP-MD) simulation and the PIMD with primitive propagator, and
adiaticity is well kept in both: the electronic kinetic energy EKINC fluctuates
with upper limit 4*10^-4 Hartree. Then I try PIMD with staging mode propagator,
but EKINC drifts straightly up to some 0.01 Har. What's the problem? The main
parameters are: EMASS=340 au, T=150 K and 300 K, PIMD # of beads P=32 for 150 K
and 16 for 300 K.

  According to the manual, PIMD must have electron thermostat, because the
frequency of inter-beads oscillations is quite high. However, my primitive
propagator simulation runs just ok without it. On the other hand, it seems that
the staging mode simulation indeed needs electron thermostat. But the question is
how to set the target EKINC? 

  The manual says: "A good choice for the frequency of the electron thermostat can
be made based on the maximum frequency in the phonon spectrum. The frequency of
the electron thermostat should be at least 2-3 times this value to avoid coupling
between the ions and the electron thermostats. As an example, for silicon, the
highest frequency in the phonon spectrum is 0.003 a.u., so a good choice for the
electron thermostat frequency is 0.01 a.u." Well, this is the frequency of the
electron thermostat, then what's its energy?

  In general, I feel uncomfortable with an electron thermostat for an insulator,
since it adds one more parameter (and source of error). As another choice, is
BO-MD more favorable in this kind of situation? It can get rid of all the trouble
with electron mass and thermostat, but how slower it is than CP-MD? Probably it is
more efficient to do PI ab initio MC instead of PIAIMD after all ...

  Thank you very much!

  Lan-Feng




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Dr. Lan-Feng Yuan,
Associate Professor,
Hefei National Laboratory for Physical Sciences at the Microscale,
University of Science and Technology of China,
Hefei, Anhui 230026, People's Republic of China
Email: yuanlf at ustc.edu.cn
Telephone: 86-551-3600994
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