Subject: Summary) QCISD/LanL2DZ

From: "Telkuni Tsuru" <telkuni..at..venus.dti.ne.jp>
Date: Thu, 3 Mar 2005 21:43:30 +0900

Hello, CCLers.

I sent following question a few days ago.

> I like to use QCISD/LanL2DZ for geometry optimization of heavy 
> rare gas atoms system on Gaussian98W.
>  example: 
> 
>     # QCISD/LanL2DZ Opt
> 
>     Opting
> 
>     Ar             0.000000      0.000000     0.000000
>     Xe             5.000000     0.000000      0.000000
>  
> When I calculate with QCISD/LanL2DZ, what things should I take 
> into account ?
> When I receive replies, I will summarize them.

I've receive two good replies. Thank you so much Dr. Per-Ola Norrby 
and Dr. Tanja van Mourik. 
Their advices include common notice to consider BSSE. And, with Norrby's 
words, I must find out suitable basis set. 

<<<<< Summary  >>>>>
1) from Per-Ola --------
Mainly that all your results will be seriously wrong, since 
the basis set is much too small for a correlated calculation.  I 
assume you want to see the vdW interaction, and it will be completely 
overshadowed by the BSSE.

And no, I don't have a good basis set, you'll have to look 
for that in the literature.

Best regards,

Per-Ola Norrby

2) from Tanja --------
Please note that the BSSE is likely going to be large for a system
like this, and will affect the equilibrium distance. It would therefore
be better to optimise with counterpoise corrections.

With Gaussian03 this can be done automatically (using the Counterpoise
option), but with Gaussian98 you'd have to do it by hand. Just calculate
a counterpoise-corrected potential energy curve by calculating the 
counterpoise-corrected Ar-Xe interaction energy at a range of different 
Ar-Xe distances. See for example:

T. van Mourik, A.K. Wilson, K.A. Peterson, D.E. Woon and T.H. Dunning,
Adv. Quant. Chem. 31, 105-135 (1999) or 

T. van Mourik, A.K. Wilson and T.H. Dunning, Mol. Phys. 96, 529-547 (1999).

Hope this helps,

Tanja

  Sincerely yours,
------------------------------------------------------
       Telkuni Tsuru     telkuni..at..venus.dti.ne.jp