[CPMD-list] forces, convergence and gga

[Axel Kohlmeyer]

On Mon, 5 Dec 2005, De Dobbelaere Christopher wrote:

CD> Hi,
CD>  
CD> Thanks for the replies (Axel also).
CD>  
CD> Referring to question 2: 
CD> Maybe i didn't formulate the problem as it should. What i did is the following:
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).

please note, that there is a significant difference between BLYP 
and B3LYP, the former does not include the hartree-fock exchange.
so you should first compare (in gaussian) how the values change 
when going from B3LYP to BLYP.


CD> Values for S-N going from 20 to 45 Ry (steps of 5 Ry) in Angstrom:
CD> 1.72176	 1.71772	 1.71836	 1.71851	 1.71658	 1.71772	
CD> Values for C-N:
CD> 1.81367	 1.82001	 1.81453	 1.82000	 1.81888	 1.82267	
CD> Values for C=C:
CD> 1.42900	 1.41553	 1.41521	 1.41571	 1.41532	 1.41554	
CD>  

CD> The question now is either or not the S-N and C-N values may be
CD> considered as converged or not? And maybe someone knows why the S-N
CD> and C-N values are not as good converged as the C=C and C-H values.
CD> We do know from experience that C-N and S-N (in our molecule) are
CD> difficult to converge in gaussian calculations also, so maybe it's
CD> just an functional issue, but maybe i'm just doing something wrong
CD> with cpmd..

what kind of _density_ cutoff did you use here? also what was
the convergence for the geometry optimization and the 
corresponding wavefunction optimization convergence parameter?

it may simply be, that you are a victim of 'ripples'.
please note, that the pseudopotentials you are using were
'optimized' to give good results at a plane wave cutoff 
of 25ry.

axel.

CD>  
CD> With kind regards,
CD>  
CD> Christopher
CD> Undergrad. Student
CD> University of Antwerp
CD>  
CD>  
CD> 
CD> ________________________________

-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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