Hi fellows, I am exploring the Car Parrinello dynamics for a system with hydrogens: CH3NH3PbI3. With N-H vibrations faster than 3000 cm^-1 and a fluctuating bandgap that may be as small as 1.5 eV , I guess I will need to use a thermostat for the electrons, in order not to use a very small emass and dt. Estimating a minimum electronic frequency sqrt(2Eg/emass), using the default emass=400, I get omega_min~0.016 atomic units. For the N-H vibrations, the frequency is 3000 cm^ -1~ 0.014 atomic units . I read elsewhere than a good frequency for the electron thermostat is 2-3 times the maximum phonon frequency (90THz), this led me to the value fnosee=270.0 My guess is 270 times larger than the default value fnosep=1.D0. Running in a parameter space different to what is tested is generally not a good idea. Am I missing something ?
A related, but independent question is about the ion thermostat. The vibrational frequencies of this system are very well separated: ~3000 cm^-1 for N-H and C-H, 900-1200cm^-1 for CH3NH3 molecular vibrations, and <300 cm^-1 for others. Hence, should I set an intermediate fnosep=30.0 (1000 cm^-1), or should I set a Nose-Hoover chain with frequencies close to every phonon band, e.g., fnosep=90.0 30.0 10.0 3.0 Thanks a lot, Eduardo Menendez Proupin University of Chile www.gnm.cl/emenendez
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