From chemistry-request*- at -*server.ccl.net Wed Jul 19 14:44:29 2000 Received: from mailhost.msi.com (fire.msi.com [146.202.0.242]) by server.ccl.net (8.8.7/8.8.7) with ESMTP id OAA00653 for ; Wed, 19 Jul 2000 14:44:27 -0400 Received: (from daemon ^%at%^ localhost) by mailhost.msi.com (980427.SGI.8.8.8/970903.SGI.AUTOCF) id LAA04833; Wed, 19 Jul 2000 11:40:51 -0700 (PDT) Received: from unknown(146.202.29.179) by mailhost.msi.com via smap (V2.0) id xma004826; Wed, 19 Jul 00 11:40:35 -0700 Message-Id: <4.2.0.58.20000719113008.00aa16c0 |-at-| 146.202.6.218> X-Sender: nxg $#at#$ 146.202.6.218 X-Mailer: QUALCOMM Windows Eudora Pro Version 4.2.0.58 Date: Wed, 19 Jul 2000 11:49:48 +0100 To: "Dr. Peter Burger" , CHEMISTRY[ AT ]ccl.net From: nxg Subject: Re: CCL:DFT & metal-metal bond distances In-Reply-To: Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; format=flowed 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: >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 > > > >-= This is automatically added to each message by mailing script =- >CHEMISTRY \\at// ccl.net -- To Everybody | CHEMISTRY-REQUEST \\at// ccl.net -- To >Admins >MAILSERV.,at,.ccl.net -- HELP CHEMISTRY or HELP SEARCH >CHEMISTRY-SEARCH#* at *#ccl.net -- archive search | Gopher: gopher.ccl.net 70 >Ftp: ftp.ccl.net | WWW: http://www.ccl.net/chemistry/ | Jan: jkl.,at,.ccl.net > > ___________________________________ 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: nxg /at\msi.com Web: http://www.msi.com