Subject: CCl: RE: SCF convergence criteria

From: "Dillen Jan <jlmd*at*sun.ac.za>" <JLMD*at*sun.ac.za>
Date: Wed, 3 Sep 2003 21:49:09 +0200

Noel,

If you are interested in the energy,
your just want 1 number with reasonable
accuracy.

If your want to optimise the geometry,
you need 1st derivatives of the energy
w.r.t. the coordinates, in other words
the change of the energy relative to a 
(very) small change in those coordinates.
Hence you need that energy to be calculated
more accurately because otherwise that
relative change may be lost "in the noise".

Although this explanation implicitly
refers to numerical derivatives, the same
argument holds for analytical ones.

Groeten
Jan

> -----Original Message-----
> From: Noel O'Boyle [mailto:noel.oboyle2*at*mail.dcu.ie]
> Sent: 03 September 2003 18:48
> To: chemistry*at*ccl.net
> Subject: CCL:SCF convergence criteria
> 
> 
> Hello all,
> This is a Gaussian 03 question.
> 
> Does anybody know or understand why the default SCF 
> convergence criteria
> for single point calculations is looser than the default for 
> geometry optimisations?
> 
> I would have expected them either to be the same for both 
> (for consistency)
> or the opposite way around (since you can afford to spend 
> more time on a
> single point convergence).
> 
> In short, if you geometry optimise a structure, surely you 
> should use SCF=Tight
> if you do a subsequent single point energy calculation. (Of 
> course, you
> can avoid this question if you use Guess=Read.)
> 
> Thanks,
> Noel O'Boyle
> 
> 
> 
> 
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