[CPMD-list] forces, convergence and gga

[De Dobbelaere Christopher]

Hi,
 
Thanks for the replies (Axel also).
 
Referring to question 2: 
Maybe i didn't formulate the problem as it should. What i did is the following:
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).
 
Values for S-N going from 20 to 45 Ry (steps of 5 Ry) in Angstrom:
1.72176	 1.71772	 1.71836	 1.71851	 1.71658	 1.71772	
Values for C-N:
1.81367	 1.82001	 1.81453	 1.82000	 1.81888	 1.82267	
Values for C=C:
1.42900	 1.41553	 1.41521	 1.41571	 1.41532	 1.41554	
 
The question now is either or not the S-N and C-N values may be considered as converged or not? And maybe someone knows why the S-N and C-N values are not as good converged as the C=C and C-H values. We do know from experience that C-N and S-N (in our molecule) are difficult to converge in gaussian calculations also, so maybe it's just an functional issue, but maybe i'm just doing something wrong with cpmd..
 
With kind regards,
 
Christopher
Undergrad. Student
University of Antwerp
 
 

________________________________

From: Juerg Hutter [mailto:hutter at pci.unizh.ch]
Sent: Mon 5/12/2005 11:25
To: De Dobbelaere Christopher
Cc: cpmd-list at cpmd.org
Subject: Re: [CPMD-list] forces, convergence and gga



Hi
> I've got 3 little questions..
>
> 1) I'm doing a md run of a molecule surrounded by water molecules. In order to check if the forces acting on the molecules are not too large, i'd like to check out the forces acting on them (since i didn't do a geometry optimalization at the beginning). I checked out the manual and found that it is possible to print the acting forces during the trajectory in the TRAJECTORY file. Since the md run is 'finished' it seems to be a little late for that..
> But now I was wondering what the initial forces acting on the atoms would be. Since they are listed in the output file that seemed not to be a problem. But what are the units of the forces? They are not mentioned in the manual, so i suppose it's a.u.? If it it, what's the conversion to convert them to dyne, Newton,.. or something else?

Forces are in a.u., I'm sorry but I'm too lazy to do the work for
converting into other units for you.

BTW. You can calculate the forces on the atoms from the trajectory file
using positions and velocities by inverting the velocity Verlet
algorithm.

>
> 2) Before I did a md run, i checked out some dft functionals to describe the geometry of the molecule. I got convergence for C=C, C-H, more or less convergence for S-N but the C-S bound  seems not to convergence at all (the C's from a aromatic system and C is attached to it). Does anyone have an idea why that could be?
>
What do you mean by convergence? All functionals give the same values?
As there is no series of functionals that will give better and better
results you cannot talk about convergence in this case.

It might be a good idea to get a "correct" value for the problematic
bond (e.g. by a large basis set CCSD(T) calculation) and then look for
the functional that comes close to that value.


> 3) In the manual, the GGA functional is mentioned. But what's the exact functional behind that GGA implemented in cpmd? Since GGA stands for Generalized-gradient-approximation, it doesnt give me any new information..
>

The reference to the papers is usually written in the output for all
functionals. You can also have a look into the file "functionals.F".

regards

Juerg Hutter



> Thanks in advance!
>
>
>
> Christopher
> Undergrad. Student
> University of Antwerp
>
>
>


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