Re: CCL:DFT & metal-metal bond distances
- From: nxg <nxg
- Subject: Re: CCL:DFT & metal-metal bond distances
- Date: Wed, 19 Jul 2000 11:49:48 +0100
What you are seeing is normal when you use
GGA (B3LYP, BP86, PW91...) exchange-correlation functionals.
These typically overestimate bond distances for molecules
and lattice constants (in the case of solids).
Now you have to be careful when you use quantum chemical
ECP's with these calculations. The reason being, these ECP's are
normally calculated using Hartree-Fock and therefore one has an
incompatibility in the exchange-correlation...i.e. you have an Hartree-Fock
ECP and a GGA exch-corr in the main calculation. Its important to
keep the theory levels consistent i.e. you should use a GGA
optimised pseudopotential if you're using that GGA in your main
You could try this.
In your case where you've been using ECP's in conjunction with your
DFT calculations, you should still see an over estimation. If you
correct answer its purely by chance. It would in some sense be the
right answer for the wrong reason...
Hope this helps
At 07:50 PM 7/19/00 +0200, Dr. Peter Burger wrote:
I got a question concerning DFT & metal-metal bond distances.
With both B3LYP and the BP86 functionals I do observe siginificantly
longer metal-metal multiple bonds compared to the experimental data
(accurate) in geometry optimizations of dinuclear (tungsten &
molybdenum) complexes, which have bond orders of 2. (M-M distance 2.72
X-ray vs 2.88 A by DFT) I have done alot calculations with different ECPs,
all electron basis for Mo, basis sets (double and triple zeta for the
metal centers) but could not cure the problem. The complexes are
dicationic and I am not including the anion so far.
The calculations are done without solvation, i.e. in the gasphase.
Comments? Anyone with an idea? Perhaps I should include point charges
in the geom. opt. step?
Many thanks in advance
University of Zuerich
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