Subject: CCL: Periodic AM1 calculations?

From: Tony Dyson <[email-masked]>
Date: Thu, 6 Jul 1995 09:53:23 +0000 (CUT)

I am attempting to model the chemisorption of small hydrocarbon 
molecules, such as ethyne, on vicinal Si(001). I don't have the 
computational resources available in order to perform any kind of ab 
initio study of this system, due to the large size of the unit cell 
required to model a surface step. Cluster calculations with AM1 have 
proven frustrating, because of course the edge effects far outweigh any 
effects of the surface step itself. At present I am in the process of 
trying out "Siligen" atoms to terminate the bonds at the edges of the 
cluster, as proposed by Redondo et al. J. Vac. Sci. Technol. 19 (1981) 
498. (See also Lutrus et al. Phys. Rev. B 48 (1993) 15086; Wu & Carter, 
Phys. Rev. B 45 (1992) 9065.)

However, what would be preferable is to perform a calculation taking into 
account the 2-dimensional periodicity of the system. In the past our 
group has done this with our own SLAB-MINDO method, eg Craig & Smith, 
Surf. Sci. 276 (1992) 174; Surf. Sci. 225 (1990) 225, and more recently 
with the CRYSTAL program of Dovesi and co-workers. The MINDO formalism 
has unfortunately passed its "use-by date" due to the availability of 
much more accurate models, while the computational and storage demands of 
CRYSTAL calculations (with reasonable basis sets) are too great for 
successful application to this problem.

It has come to my attention that there is a periodic implementation of 
the AM1 scheme contained within MOPAC 93. Do any of the CCL contributors 
have experience with this version of MOPAC? I would like to find out 
whether 2-dimensional periodicity is implemented or whether the technique 
is restricted to full 3-dimensional periodicity.

Thank you,
	Tony Dyson

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  Mr. Anthony J. Dyson		[email-masked]
  Dept. of Physics		phone: +49 21 5425
  University of Newcastle	fax:   +49 21 6907
  Callaghan, Australia, 2308

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