Subject: CCL:A BLYP/Becke3LYP frequency problem in G92/DFT - Summary

From: <[email-masked]>
Date: Sun, 7 May 1995 07:57:26 +1000 (EETDT)

Some time ago I posted the following problem about getting poor "zero"
frequencies with G92/DFT.
 
> I have been using BLYP and Becke3LYP for some large systems where
> MP2 frequencies are not feasible here with some success. With one system 
> I have a problem. The molecule has D2h symmetry and is like diborane.
> Experimentally we know this but are using theory to help the neutron
> diffraction studies and to sort out the i.r./Raman bands.
> 
> SCF gives reasonable results with two basis sets. BLYP and Becke3LYP
> give similar geometries to the SCF but the SCF gives one imaginary
> frequency. However the 3 "zero" rotational frequencies are roughly
> -30, -20 and -8. The imag freq is -40. I used opt=tight and Int=FineGrid.
> The frequency run still shows the forces and predicted displacements
> to be good. 
> 
Several people offered suggestions. The most comprehensive reponse
was from Benny Johnson:_

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The main reason for nonzero DFT rotational frequencies in Gaussian is the
lack of proper rotational invariance in the quadrature grid.  This is 
discussed in 

B. G. Johnson and M. J. Frisch
Chem. Phys. Lett. 216, 133 (1993).

and a rigorous solution is given in

B. G. Johnson, P. M. W. Gill and J. A. Pople
Chem. Phys. Lett. 220, 377 (1994).

Hope this helps,

Benny Johnson
Q-Chem, Inc.

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I also thank Ross Nobes and Tony Scott for help.

The problem can be overcome, at least with G94. It is important to
use opt=tight or opt=verytight and a fine grid. With G92/DFT the finest
grid is obrained with Int=FineGrid. For my case this was not good enough.
For G94 Int=FineGrid is now the default and various finer grids are
possible. I was able to get good results with a finer grid. I thinh
the details are in the manual but I do not have a copy. Ross Nobes
gave me the details.

A proper fix is coming out in QChem.

I think that summarises the replies that pointed to the problem.

Cheers, Brian.
-- 
        Associate Professor Brian Salter-Duke (Brian Duke)
School of Mathematical and Physical Sciences, Northern Territory University,
            Darwin, NT 0909, Australia.  Phone 089-466702
e-mail: [email-masked]  or [email-masked]