[CPMD-list] What are classic energies for CP MD and PI MD?

[Lan-Feng Yuan (lyuan@Princeton.EDU)]

  Dear Axel,

> LF>   I am using CPMD to do path integral simulations and find it very
> LF> useful, thanks! The total quantum energy in output corresponds to
> LF> the total energy estimator, and by subtracting the nuclear ground
> LF> state energy, I can get the vibrational energy at a given
> LF> temperature. Well, what is the "ECLASSIC" in output? Also, in a
> 
> i can't speak for the PI-MD case, but ...
> 
> LF> classic CP MD simulation, "ECLASSIC" is also printed. Are they the
> LF> same thing? I guess it's the sum of Kohn-Sham energy and classic
> LF> nuclear kinetic energy (which is proportional to instant
> LF> temperature). If this is correct, then ECLASSIC - EKS should be
> LF> equal to 3/2*N*k_B*T (N is number of atoms) when average 
> temperatureLF> reaches its target value, right? However, it seems 
> that I never get
> LF> it exact. What's the problem? Thank you very much!
> 
> are you doing an isolated system (symmetry 0)? 
> then you have only 3N-3 DOFs.
> 
> axel.
> 
  Yes, I am doing symmetry 0. 3N-3? Not 3N-6? Because translation is frozen, but rotation not? Anyway, it's still not quite right. For example, for a H2O molecule at 150 K, I get:

                              MEAN VALUE       +/-  RMS DEVIATION
                                     <x>     [<x^2>-<x>^2]**(1/2)
 ELECTRON KINETIC ENERGY    0.382262E-04             0.289944E-04
 IONIC TEMPERATURE                145.36                   110.93
 DENSITY FUNCTIONAL ENERGY    -17.184344             0.132918E-03
 CLASSICAL ENERGY             -17.180351             0.289125E-04
 CONSERVED ENERGY             -17.180313             0.197829E-05

  So ECLASSIC - EKS = 0.003993 Har = 0.1087 eV = 8.40 k_B*T (k_B*T = 0.01293 eV). I can't figure out how this 8.4 comes ...

  Thank you very much! Best regards,

  Lan-Feng