[CPMD-list] forces, convergence and gga

[Juerg Hutter]

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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