Subject: ONIOM Question

From: "Cherif F. Matta" <[email-masked]>
Date: Thu, 11 Jul 2002 10:56:41 -0400 (EDT)

Dear Colleagues,

We are interested in studying localized chemical reactions on silicon 
Si(111) surfaces, a complex surface which requires large clusters for a 
good representation of the reaction environment.  The ONIOM method is 
probably our best bet to circumvert the computational bottle neck.

I would very much appreciate any help to resolve the following:

In the Si(111) cluster, there is a number of tri-coordinated Si atoms 
(each with one dangling bond). The part to be included in the high-level 
calculation does not include all of the dangling bonds.  Therefore the 
multiplicities of the high-, intermediate-, and low-levels should be 
different.

Gaussian98 seems to mandate the usage of the same multiplicity for the 
three layers. Firsttly, is that a true statement?. If not, then how can 
one specify in G98 different multiplicity for different layers? 

The following is a section of an input file (which failed):

#p oniom=(ub3lyp/3-21g*:ub3lyp/Gen:am1) opt=z-matrix

Si(111), 3 ad-atoms + 1 rest atom with 1 Cl-Benz adsorbed

0 2	 	(multiplicity for the H level - only 1 dangling bond)
0 3 		(multiplicity for the M level - 2 dangling bonds)
0 3 		(multiplicity for the L level - 2 dangling bonds)
 Si 0  x1  y1  z1  M
 Si 0  x2  y2  z2  H
 Si 0  x6  y6  z6  M                
 ....... 
 .......

G98 does not allows except for ONE line for multiplicity specification 
followed by the molecular geometry. As a result the same multiplicity 
is assigned for the three layers.

Thank you very much.   

    Cherif
    
  -----------------------------------------
  Chérif F. Matta,
  Theoretical/Computational Chemist,
  Lash Miller Chemical Laboratories,
  Chemistry Department,
  University of Toronto,
  80 St. George Street (Room 261),
  Toronto, Ontario, Canada M5S 1A1.
  ..........................................
  Telephone:      (416) 978 - 3625
  Cellular:       (416) 996 - 2444
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  Web Site: http://chem.utoronto.ca/~cmatta/
  ------------------------------------------