From: adriana <[email-masked]>
Date: Thu, 27 Apr 2000 12:08:39 -0300

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?

Thank you very much in advance for your answers which
I will eventually summarize.