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

[Nicola Marzari]

Dear Ruijuan Xiao,

bulk Na is a metal, so it's very difficult to treat properly with a
straightforward Car-Parrinello approach - this is often discussed in
Car-Parrinello review papers, such as the one posted by Dominic Marx
on his webpage.

The cp code uses, for maximum efficiency, Gamma sampling only; it's
possible that at that point in the BZ a reasonable gap exists between
occupied and unoccupied states (even if, elesewhere in the BZ, these
cross). So you might be able to run a simulation for a while, but it's
a strategy that borders on disaster, as I think it is happening to you.

A safer strategy would be to begin by using PWSCF - an even better
strategy would be to go to the review literature - ideally something
like Richard Martin's electronic structure book, or taking the
electronic-structure lectures from
http://ocw.mit.edu/OcwWeb/Materials-Science-and-Engineering/3-320Spring-2005/CourseHome/index.htm

All the best,

			nicola


Ruijuan Xiao wrote:
> Dear all,
> I met some problems when I do some calculations by cp.x. What I calculated is a cubic box in which 54 Na atoms exist. When I use dt=5.0d0, emass=400.0d0, and emass_cutoff=3.0d0, it can run without any problems. When I put dt=5.0d0,emass=400.0d0, then change the value of emass_cutoff, the calculation also runs without problems.
> 
> But when I put emass=400.0d0 and emass_cutoff=3.0d0,then change dt into 10.0d0, the following error message appears and the program stopped.
> -----------------------------------
> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>      from ortho : error #        21
>      max number of iterations exceeded
>  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
> 
>      stopping ...
> ------------------------------------
> 
> When I put dt=5.0d0 and emass_cutoff=3.0d0, but change emass into 200.0d0, the program stopped after 12 iterations. The message is :
> ------------------------------------
>   nfi    ekinc  temph  tempp        etot      enthal       econs       econt    vnhh   xnhh0    vnhp   xnhp0
>    10 ********    0.0    0.0    41.82500    41.82500    41.82500   146.23636  0.0000  0.0000  0.0000  0.0000
>    11  NaN        0.0    0.0    58.16717    58.16717    58.16717  NaN         0.0000  0.0000  0.0000  0.0000
>    12  NaN        0.0    0.0  NaN         NaN         NaN         NaN         0.0000  0.0000  0.0000  0.0000
> 
>    MAIN:          EKINC   (thr)          DETOT   (thr)       MAXFORCE   (thr)
>    MAIN:  NaN            0.1D-03 NaN            0.1D-08  0.000000D+00  0.1D+11
>    MAIN: convergence achieved for system relaxation
> 
> 
>               averaged quantities :
>          ekinc          ekin          epot          etot     tempp
>  NaN           NaN           NaN           NaN                 0.0
> -----------------------------------
> It seems that the calculation is sensitive to these parameters. Since on the page 37 of the maunal, it says that "unless you are already experienced with the system you are studying or with the code internals, usually you need to tune some
> input parameters, like emass, dt, and cut-offs.", so these parameters must be important for calculations, but now I am puzzled about how to adjust emass,dt and cutoff in cp calculations.Would you like to give me any information or any suggestion about that?
> 
> Thank you very much.
> 
> 
> Best regards,
> 
> Sincerely,
> Ruijuan Xiao
> Institute of Physics,
> Chinese Academy of Sciences
> 
> 
> 
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---------------------------------------------------------------------
Prof Nicola Marzari   Department of Materials Science and Engineering
13-5066   MIT   77 Massachusetts Avenue   Cambridge MA 02139-4307 USA
tel 617.4522758 fax 2586534 marzari at mit.edu http://quasiamore.mit.edu