Subject: CCL: DFT & metal-metal bond distances

From: "Dr. Peter Burger" <[email-masked]>
Date: Wed, 19 Jul 2000 19:50:44 +0200

Dear CCLers,

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

Peter
------------------------------------
Peter Burger
University of Zuerich

---

From: nxg <[email-masked]>
Date: Wed, 19 Jul 2000 11:49:48 +0100

Dear Peter,

    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 
calculation.
    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 do 
get the
    correct answer its purely by chance. It would in some sense be the
    right answer for the wrong reason...

    Hope this helps

  -Niranjan

At 07:50 PM 7/19/00 +0200, Dr. Peter Burger wrote:
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___________________________________
Niranjan Govind, 
Ph.D.
Scientist, Materials Science
Molecular Simulations, Inc
9685 Scranton Road,  CA 
92121-3752
Tel:   858-799-5337, Fax:  858-458-0136
Email:  [email-masked]
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