From:	Josh Bowden <9702029k { *at * } lv.levels.unisa.edu.au>
Date:	Tue, 14 Jul 1998 10:33:37 +0930
Subject:	Summary : Mixing basis sets.

Hello all,
Thanks to you who replied.

I wrote :
>As a continuation of recent discussion on this list about 3-21G vs 6-31G*, can I
ask if anyone
>has information on mixing basis sets in a calculation. Programs such as CRYSTAL
95 and
>HyperChem (probably others as well) allow you to apply different basis to
individual atoms. If >the lack of polarization functions ( as stated by cory
at.at chem.ucalgary) is a problem can you >not apply a bigger basis set to
'problem' atoms and reduce the basis of others?
>Has anyone looked at this systematically and is there any referencs on this
topic?
>Are there any problems in this metholodgy (does it have any major downfalls)?


Dr. Cory C. Pye (cory;at;zinc.chem.ucalgary.ca) wrote :
>The biggest problem with mixing basis sets is that you run the risk of
>obtaining an 'unbalanced' overall basis set. This basically means that the
>electronic distribution, compared to the true distribution, is lopsided.
>A way to think of this is that if you have a homonuclear diatomic with a
>minimal basis set on one atom (A) and a triple zeta valence + pol + dif on
>the other atom (B), then, because B has more functions over which to distribute
>electron density, you will basically have a dipole moment for this homonuclear
>diatomic (more charge on B than A), which is physically unrealistic.
>
>One should therefore always try to balance the basis set by using similar
>types on different atoms. For example (from my own work) in
>J. Org. Chem., 60, 2328-9 (1995), J. Org. Chem., 63, 105-112 (1998), and
>J. Phys. Chem B, 102, 3564-3573 (1998), we did not have available a
>6-31G* basis set for atoms of interest beyond Ar (Br,I,Se,Te,As,Sb,Ge,Sn(x2);Cd)
>so we spliced in a Huzinaga basis set (from his book ---
>take the minimal basis set with the highest # of primitives in the innermost
>shell; split the valence shell (n) to ( (n-1)(1) ); and add a polarization
>function provided) to give a SVP type basis set (like 6-31G*).
>
>On a related note, there is a basis set called 3-21G(N) (by Hehre, I believe)
>where the 3-21G* basis set is augmented by nitrogen polarization functions.



Dr. Georg Schreckenbach (schrecke[ AT ]t12.lanl.gov) wrote :
>In any case, I think people have used such "locally dense" basis sets a
>lot. Typically, you would use the high level basis set on, e.g., the metal
>atom AND its nearest neighbors, and smaller basis sets elsewhere. One
>reference is a review by Don Chesnut: D. B. Chesnut, in Annual Reports on
>NMR Spectroscopy Vol. 29, Academic Press 1994, p. 71. He uses such locally
>dense basis sets for NMR calculations, and I believe that he has newer
>papers on this as well.
>        The more recent NMR review by de Dios has similar information (A.
>C. de Dios, Prog. NMR Spectros. 29 (1996), 229.) You will find further
>references in these reviews.
>
>Furthermore, and this is slightly off from your question, the idea of using
>a higher level of theory for the interesting atoms, and a lower level
>elsewhere, has been pursued in the QM/MM methods: In this case, you do
>whatever quantum mechanical method you need at the reaction center, and
>molecular mechanics at the bulky ligands or solvent or so. The Morokuma
>version of this ("ONIOM") is present in GAUSSIAN98, as far as I know.
>Others have done such things as well, e.g. T.K.Woo/T.Ziegler.
>
>Best regards, Georg



Robert J. Zellmer (rzellmer;at;chemistry.ohio-state.edu)
wrote :
>I think most programs that allow you to put in general basis sets would be
>able to do this.  I know GAMESS is capable of this and I have done it using
>GAMESS.  I don't know of any systematic study of doing this, at least not one
>that has been published.  I know a few years back when I was doing some
>calculations on perfluoroethers in conjuction with someone at Dupont they had
>determined that d-type polarization functions could be left off the Fluorines
>w/o any real affect on the calculations, but they were still needed on the
>Oxygen atoms.  This might be mentioned in some of the papers published by the
>folks at Dupont and I can check if you like.
>


Stefan Konietzny (konietz -x- at -x- chemie.uni-kl.de) wrote :
>Hi!
>
>I dont know which program you use, but in GAUSSIAN it is possible.
>
>Try the GEN Keyword instead of of general basis set.
>
># hf/gen fopt ....
>
>After the molecule specification you have to apply the basis to the atoms.
>You can do this like this
>
>H C 0
>6-31+g(d)
>****
>
>This means every H and C atom has 6-31+g(d).
>
>But if you use the numbers of the atoms, you can apply different basis
>sets to different H (or C, or anything else) atom.
>
># hf/gen fopt name=konietz
>
>h2o
>
>0 1
>h
>o 1 1.1
>h 2 1.1 h 108.
>
>2 0
>6-31g
>****
>1 3 0
>3-21g
>****
>
>Hope this helps.
>

Thanks again,

Josh

Josh Bowden
Ian Wark Research Institute
University of South Australia
E-mail : 9702029k.,at,.lv.levels.unisa.edu.au




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