Sorry for replying to this old thread. I still don't quite understand
the behavior for the energy as a function E_{cut,psi} for the
case of E_{cut,rho}=4*E_{cut,psi}. You would expect for a
norm-conserving pseudopotential that the total energy be monotonically
decreasing (Rayleigh-Ritz principle) as it achieves convergence. But
adding the non-linear core correction (nlcc) seems to negate
this behavior. I am assuming that the nlcc can be evaluated exactly in
real-space. Perhaps the FFT is leading to some aliasing
effect?On Thu, Oct 20, 2011 at 11:05 AM, William Parker <wparker at anl.gov> wrote: > Thanks, Eric, for the insight. ?That core correction can be vicious. ?Turning > up the density cutoff does help. ?There is still oscillation in the total > energy at the 100 micro-Ry (meV) level for density cutoff 16 times the wave > function cutoff, but I guess one shouldn't expect too much stability at that > level. > > https://wiki.alcf.anl.gov/wparker/images/6/68/ZnO_ecut_convergence_rho4_10_16_lda_ncpp.png > > --William > > On Oct 19, 2011, at 9:13 PM, Eric J. Walter wrote: > >> >> Hi, >> >>> Just to double-check. Is charge density cutoff 4 times that planewave >>> cutoff? >> >> I think that this hits on the issue... ?The Zn.pz-nc.UPF has a sharp >> core correction. ?The FFT/Density grid >> needs to be increased significantly, here is a plot of from 4x to 20x >> ecutwfc for the case with and without >> a core correction in Zn. I think this is the cause of the odd *ecutwfc* >> shown by William Parker. >> >> http://dl.dropbox.com/u/8094696/Znplot.pdf >> >> So, in this case, no, the density cutoff needs to be higher: >> >> >> Eric >> >> >> >> >> On 10/19/2011 08:47 PM, Nichols A. Romero wrote: >>> The behavior here is what I might expect for a real-space DFT code, >>> but V_NL is evaluated in reciprocal space right (we don't have >>> real-space projectors)? >>> >>> Just to double-check. Is charge density cutoff 4 times that planewave >>> cutoff? >>> >>> >>> On Wed, Oct 19, 2011 at 2:51 PM, William Parker<wparker at anl.gov> ?wrote: >>>> Dear Yun Song and interested parties, >>>> >>>> The possibility of a phonon test case in ZnO piqued my curiosity so I >>>> decided to have a go at it. ?I find a very odd convergence pattern with >>>> these two pseudopotentials in this system--the total energy has a damped >>>> sinusoidal decay with increasing cutoff: >>>> (selecting the maxima and minima at 10 Ry sampling) >>>> >>>> E_cutoff (Ry) ? E_total (Ry) >>>> 100 ? ? ? ? ? ? -338.741 >>>> 160 ? ? ? ? ? ? -338.660 >>>> 210 ? ? ? ? ? ? -338.675 >>>> 290 ? ? ? ? ? ? -338.665 >>>> 370 ? ? ? ? ? ? -338.668 >>>> 400 ? ? ? ? ? ? -338.667 >>>> >>>> Figure here: >>>> https://wiki.alcf.anl.gov/wparker/images/b/b5/ZnO_ecut_convergence_lda_ncpp.png >>>> Details here: >>>> https://wiki.alcf.anl.gov/wparker/index.php/Zinc_Oxide_Parameter_Convergence >>>> >>>> I have never encountered this kind of behavior with norm-conserving >>>> pseudopotentials before. ?At worst, I've seen little steps in an otherwise >>>> smooth convergence to a value. ?The eigenvalues at the various k-points >>>> don't appear to have any ghost states that come and go with cutoff. ?Has >>>> anyone observed this kind of oscillation in the total energy with >>>> increasing plane-wave cutoff before? ?What gives rise to it? >>>> >>>> --William >>>> >>>> ********************************************************* >>>> ?William D. Parker >>>> ?Computational Postdoctoral Fellow ? fax: (630) 252-4798 >>>> ?MSD-212, Rm. C-215 >>>> ?Argonne National Laboratory >>>> ?9700 S. Cass Ave. >>>> ?Argonne, IL 60439 >>>> ********************************************************* >>>> >>>> >>>> >>>> >>>> _______________________________________________ >>>> Pw_forum mailing list >>>> Pw_forum at pwscf.org >>>> http://www.democritos.it/mailman/listinfo/pw_forum >>>> >>> >>> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum > > ********************************************************* > ?William D. Parker > ?Computational Postdoctoral Fellow ? fax: (630) 252-4798 > ?MSD-212, Rm. C-215 > ?Argonne National Laboratory > ?9700 S. Cass Ave. > ?Argonne, IL 60439 > ********************************************************* > > > > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > -- Nichols A. Romero, Ph.D. Argonne Leadership Computing Facility Argonne, IL 60490 (630) 447-9793
