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<TITLE>RE: [CPMD-list] forces, convergence and gga</TITLE>
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<P>Hi cpmd users,</P>
<P>Another few additions :)</P>
<P>> CD> Hi,<BR>> CD> <BR>> CD> Thanks for the replies (Axel
also).<BR>> CD> <BR>> CD> Referring to question 2:<BR>> CD>
Maybe i didn't formulate the problem as it should. What i did is the
following:<BR>> CD> I want to use the VDB uspp and BLYP functional
(because B3LYP describes the molecule relatively good in gaussian). So i tried
different cutoff values (20 to 45) and looked for convergence for some
'important' bond lengths. I think the convergence for S-N and S-C is not really
good, but C=C and C-H seem to converge nicely (cutoff values going from 20 Ry to
45 Ry).<BR>><BR>> please note, that there is a significant difference
between BLYP<BR>> and B3LYP, the former does not include the hartree-fock
exchange.<BR>> so you should first compare (in gaussian) how the values
change<BR>> when going from B3LYP to BLYP.</P>
<P><BR>I know, but I'm not worried about the fact that the values are 'wrong',
it's just that they do not converge when increasing cutoff values that worries
me.</P>
<P><BR>> CD> Values for S-N going from 20 to 45 Ry (steps of 5 Ry) in
Angstrom:<BR>> CD> 1.72176
1.71772
1.71836
1.71851
1.71658
1.71772 <BR>> CD> Values for
C-N:<BR>> CD> 1.81367
1.82001
1.81453
1.82000
1.81888
1.82267 <BR>> CD> Values for
C=C:<BR>> CD> 1.42900
1.41553
1.41521
1.41571
1.41532
1.41554 <BR>> CD> <BR>> <BR>>
CD> The question now is either or not the S-N and C-N values may be<BR>>
CD> considered as converged or not? And maybe someone knows why the
S-N<BR>> CD> and C-N values are not as good converged as the C=C and C-H
values.<BR>> CD> We do know from experience that C-N and S-N (in our
molecule) are<BR>> CD> difficult to converge in gaussian calculations
also, so maybe it's<BR>> CD> just an functional issue, but maybe i'm just
doing something wrong<BR>> CD> with cpmd..<BR>><BR>> what kind of
_density_ cutoff did you use here? also what was<BR>> the convergence for the
geometry optimization and the<BR>> corresponding wavefunction optimization
convergence parameter?</P>
<P><BR>I used the following parameters:</P>
<P> CONVERGENCE ORBITALS<BR> 1.0d-7<BR> CONVERGENCE
GEOMETRY<BR> 4.5d-4</P>
<P>CONVERGENCE INITIAL<BR> 1.0d-4<BR> CONVERGENCE ADAPT<BR>
0.02<BR> CONVERGENCE ENERGY<BR> 0.05</P>
<P>and</P>
<P> GC-CUTOFF<BR> 1.0d-06</P>
<P><BR>> it may simply be, that you are a victim of 'ripples'.<BR>> please
note, that the pseudopotentials you are using were<BR>> 'optimized' to give
good results at a plane wave cutoff<BR>> of 25ry.</P>
<P><BR>I thought the best way to get a 'good' cutoff value is to try several
ones and get use the one that gives convergence (more or less)?</P>
<P> </P>
<P>Thanks in advance,<BR>Christopher<BR>Undergrad. Student<BR>University of
Antwerp</P>
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