[CPMD-list] geometry optimization - isotopes and convergence doubt

[Axel Kohlmeyer]

On Mon, 31 Mar 2008, Mark Kosmowski wrote:

MK> Dear CPMD Community:

dear mark,

MK> I am taking the advice given earlier by Axel and running some
MK> convergence vs. cutoff testing and have a couple questions.  I will
MK> briefly describe my calculations and then ask my questions.

that is all good and well, but let me also remind you of the
problem of trying to get an accuracy, that the methods you use 
by themselves do not provide. i.d. using DFT, pseudpotentials 
and finite (fft compatible) grids.

[...]

MK> At this cutoff, I found that I needed to use (minimally) 2 k points in
MK> each dimension to achieve zero negative frequencies, so I am
MK> performing the convergence vs. cutoff energy using this k point
MK> condition.  I am now running additional tests at 50, 100, 200 and 250
MK> Ry to examine geometry and vibrational convergence.  Is this a proper
MK> strategy?

yes. however, you are not mentioning the convergence for the geometry
optimization which in turn depends on the (final) convergence for the
wavefunction.


MK> At the 150 Ry cutoff, I notice a 0.00017 A bond length difference (out
MK> of 1.57 A bond length) in the bond of most interest between the 1H and
MK> 2H geometries.  There is also a 0.000244 (out of -32.65) total energy
MK> difference.  Is this expected behavior?  If so, do these differences
MK> have physical meaning?

not really. with GGA's you have a typical error of about 1% underbinding. 
i'd have to see how identical the inputs were. so trying to go far 
beyond that is somewhat pointless unless you switch to a different 
method (and thus a different code).

MK> Lastly, at the 50 Ry cutoff for the 2H geometry optimization, I am
MK> seeing several instances of the following message prior to the final
MK> geometry:
MK> 
MK>  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
MK>  !! UPDRHO| B2MAX IS VERY HIGH                                 !!
MK>  !! UPDRHO| DOUBT ON CONVERGENCE. WE TRY ONCE AGAIN            !!
MK>  !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
MK>            1     2.380E-04  -32.291104   0.000E+00   1.00    1.78
MK>            2     2.216E-04  -32.291104   6.933E-08   6.00    6.22
MK> 
MK> I am assuming that this just means that 50 Ry is not a high enough
MK> cutoff energy for my system regardless what the parameter vs. cutoff

that is more likely because of an instability in the lanzcos 
procedure and you'll have to tweak the lanzcos related parameters
to get it to run more stable. it looks also like the code detected
that and resetted the diagonalization.

cheers,
   axel.

MK> plots look like.  Is this a correct conclusion?
MK> 
MK> Thank you,
MK> 
MK> Mark Kosmowski
MK> 
MK> Graduate Student
MK> Physical Chemistry
MK> Syracuse University
MK> Syracuse, NY, United States
MK> _______________________________________________
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MK> CPMD-list at cpmd.org
MK> http://cpmd.org/mailman/listinfo/cpmd-list
MK> 

-- 
=======================================================================
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
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