[Pw_forum] How to adjust emass,dt and cutoff in cp.x calculation?

Thu Apr 13 05:03:11 CEST 2006

Dear Nicola,Dear Sandro,Dear Paul,and Dear Axel,

I am so happy to receive so much valuable information. Thank you so much.
The system that I want to study is metallic. It seems that I should be more careful with that. Now I decide to start from a nonmetallic system to learn the CP method following your instructions.After I understand the method more, I hope I can move to my metallic system correctly. 

The details you provide are really helpful. Thanks a lot.

Best regards,

Sincerely,
Ruijuan Xiao
Institute of Physics,
Chinese Academy of Sciences

----- Original Message -----

From: Axel Kohlmeyer <akohlmey at vitae.cmm.upenn.edu>
To：Paul Tangney <tangney at civet.berkeley.edu>
Sent：2006-04-13 03:49:32
Subject：Re: [Pw_forum] How to adjust emass,dt and cutoff in cp.x calculation?

>On Wed, 12 Apr 2006, Paul Tangney wrote:
>
>
>hi paul (and the others).
>
>just to add one more detail to that.
>
>[...]
>
>PT> Most people either don't know about this issue or use the
>PT> ostrich approach to computer simulation and skip step 5 but
>PT> the tests that have been published suggest that this is
>PT> a mistake.
>
>well put. 
>
>[...]
>
>PT> You could also just use Born-Oppenheimer MD, which is
>PT> always faster for a chosen level of accuracy on the forces
>PT> or the Kohn-Sham energy.
>PT> It doesn't conserve energy as well, but the energy that
>PT> is conserved in Car-Parrinello MD is a physically meaningless
>PT> quantity anyway, so who cares?
>PT> If temperature drifts too much, attach a weak thermostat.
>
>...also it actually does not drift as much if you use 
>wavefunction extrapolation with BO-MD. in my tests 4th-order
>extrapolation gives not only a _very_ good guess, i.e. reduces
>the number of iterations needed for convergence, but also seems
>to reduce the drift in kinetic energy, _even_ if you losen the 
>convergence criterion. we recently had somebody here presenting
>MD studies with gaussian 03 and he was seeing the same.
>
>regards,
>   axel.
>
>PT> 
>PT> 
>PT> Regards,
>PT> 
>PT> Paul
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> What Nicola says is entirely correct, and you should pay special
>PT> attention when studying metallic systems with CP (or seriously
>PT> considering using extensions of the CP method like the ones pioneered by
>PT> Nicola and by others).
>PT> 
>PT> However, should you still be willing to study Na with CP as a test case,
>PT> or should you consider studying "easier" (say, nonmetallic) systems in
>PT> the future, the recipe for setting emass, emass_cutoff, and dt is
>PT> roughly as follows:
>PT> 
>PT> 1) set emass to a physical sound value. The value of emass determines
>PT> how much the fictitiuous dynamics of the electrons couples with the real
>PT> dynamics of the ions. A small emass guarantees decoupling. How small is
>PT> small is typically determined by the excitation gap E_gap of your
>PT> system. The minimum frequency of the fictitious electronic dynamics
>PT> scales like sqrt(E_gap/emass) [see, e.g., Pastore et al, PRA 44, 6334
>PT> (1991)], and this has to be much higher than the maximum frequency of
>PT> the ion dynamics. Typically a factor of three larger is enough. As you
>PT> see, metallic systems, where E_gap=0 are a bit of a nightmare for CP,
>PT> unless the finite size of your system introduces a finite gap, but then
>PT> you have to be extra-careful...
>PT> 
>PT> 2) start playing with emass_ecutoff, by fixing it to a starting value,
>PT> and checking what is the maximum dt that alllows you to integrate the
>PT> equations of motion. The error you got is precisely a sign that the dt
>PT> you used is too large for the integrator to converge.
>PT> 
>PT> 3) choose the value of emass_ecutoff that allows you to use the largest dt.
>PT> 
>PT> 4) Set dt accordingly.
>PT> 
>PT> Points (2-4) are described in detail in Tassone, Mauri, Car, PRB 50,
>PT> 10561 (1994).
>PT> 
>PT> Regards,
>PT> Sandro
>PT> 
>PT> 
>PT> Ruijuan Xiao wrote:
>PT> 
>PT>  >Dear all,
>PT>  >I met some problems when I do some calculations by cp.x. What I 
>PT> calculated is a cubic box in which 54 Na atoms exist. When I use 
>PT> dt=5.0d0, emass=400.0d0, and emass_cutoff=3.0d0, it can run without any 
>PT> problems. When I put dt=5.0d0,emass=400.0d0, then change the value of 
>PT> emass_cutoff, the calculation also runs without problems.
>PT>  >
>PT>  >But when I put emass=400.0d0 and emass_cutoff=3.0d0,then change dt 
>PT> into 10.0d0, the following error message appears and the program stopped.
>PT>  >-----------------------------------
>PT>  >%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>PT>  >     from ortho : error #        21
>PT>  >     max number of iterations exceeded
>PT>  > 
>PT> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>PT>  >
>PT>  >     stopping ...
>PT>  >------------------------------------
>PT>  >
>PT>  >When I put dt=5.0d0 and emass_cutoff=3.0d0, but change emass into 
>PT> 200.0d0, the program stopped after 12 iterations. The message is :
>PT>  >------------------------------------
>PT>  >  nfi    ekinc  temph  tempp        etot      enthal       econs 
>PT>   econt    vnhh   xnhh0    vnhp   xnhp0
>PT>  >   10 ********    0.0    0.0    41.82500    41.82500    41.82500 
>PT> 146.23636  0.0000  0.0000  0.0000  0.0000
>PT>  >   11  NaN        0.0    0.0    58.16717    58.16717    58.16717  NaN 
>PT>          0.0000  0.0000  0.0000  0.0000
>PT>  >   12  NaN        0.0    0.0  NaN         NaN         NaN         NaN 
>PT>          0.0000  0.0000  0.0000  0.0000
>PT>  >
>PT>  >   MAIN:          EKINC   (thr)          DETOT   (thr)       MAXFORCE 
>PT>    (thr)
>PT>  >   MAIN:  NaN            0.1D-03 NaN            0.1D-08  0.000000D+00 
>PT>   0.1D+11
>PT>  >   MAIN: convergence achieved for system relaxation
>PT>  >
>PT>  >
>PT>  >              averaged quantities :
>PT>  >         ekinc          ekin          epot          etot     tempp
>PT>  > NaN           NaN           NaN           NaN                 0.0
>PT>  >-----------------------------------
>PT>  >It seems that the calculation is sensitive to these parameters. Since 
>PT> on the page 37 of the maunal, it says that "unless you are already 
>PT> experienced with the system you are studying or with the code internals, 
>PT> usually you need to tune some
>PT>  >input parameters, like emass, dt, and cut-offs.", so these parameters 
>PT> must be important for calculations, but now I am puzzled about how to 
>PT> adjust emass,dt and cutoff in cp calculations.Would you like to give me 
>PT> any information or any suggestion about that?
>PT>  >
>PT>  >Thank you very much.
>PT>  >
>PT>  >
>PT>  >Best regards,
>PT>  >
>PT>  >Sincerely,
>PT>  >Ruijuan Xiao
>PT>  >Institute of Physics,
>PT>  >Chinese Academy of Sciences
>PT>  >
>PT>  >
>PT>  >
>PT>  >_______________________________________________
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>PT>  >
>PT>  >
>PT>  >
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>PT> 
>
>-- 
>=======================================================================
>Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
>   Center for Molecular Modeling   --   University of Pennsylvania
>Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323
>tel: 1-215-898-1582,  fax: 1-215-573-6233,  office-tel: 1-215-898-5425
>=======================================================================
>If you make something idiot-proof, the universe creates a better idiot.
>
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