Subject: Re: UHF/6-31G*, UHF/6-311G**, PMP2/3-21G* ...

From: "Alexei Khalizov" <[email-masked]>
Date: Tue, 2 May 2000 17:07:55 -0400

Hi,

It seems that you have a competition of (i) incompleteness of basis 
set and (ii) electron correlation treatment. In some cases you have 
cancellation of errors that gives reliable results. 

Underestimated or even negative barriers are observed when spin 
projection is applied. 

You may find some useful information on this subject in recent 
book by Jensen, F "Introduction to computational chemistry", 
Chichester ;  New York :  Wiley,  1999.

Alexei

---------------
Hi CCL'ers:

I have calculated the activation energies for addition reactions 
of the oxygen atom in its ground state to halogenated olefins 
with the Gaussian 98 program by the QST3 method at the following 
theory levels: UHF/6-31G*, UHF/6-311G**, PMP2/3-21G*, 
PMP2/3-21G**, MP4SDQ/3-21G**, CCD/3-21G**, PMP2/6-31G**
and  MP4D/6-311G**. 

The results showed that: 

- the barrier values are overestimated by using 
the levels UHF/6-31G*, UHF/6-311G**, MP4SDQ/3-21G** and
 CCD/3-21G**,

- at the levels PMP2/6-31G** and  MP4D/6-311G**, the barriers 
are underestimated and even negative which is known 
to be impossible. 

- However, the activation energies calculated with PMP2/3-21G* 
and  PMP2/3-21G** are within experimental error. 

I would like to know why the more sophisticated methods 
give erroneous values?

-------------------------
Dr. Alexei Khalizov
Department of Chemistry
McGill University,
801 Sherbrooke St. W.,
Montreal, Quebec, CANADA,
H3A 2K6

Phone: (514) 398-6920
Fax: (514) 398-2382