Subject: Different mopac and gaussian AM1 results

From: Mario Tulio Rosado <[email-masked]>
Date: Wed, 17 Jan 1996 16:53:33 +0100 (LISBOA)

Dear CCL,

I started to make some semiempirical calculations (AM1 and PM3) under MOPAC 6.0
(DOS version) some time ago.
Right now i had to repeat some of them and i did it under gaussian.
This is what happened:

The ENERGY VALUES after optimization that led exactly to the same structure
(Glycine SCF-RHF/AM1) didn't match the earlier MOPAC results.
I used the "HF" value given by gaussian in hartrees and compared to the
"Total Energy" given by MOPAC in eV. The "entalpy of formation" value didn't
match also (this one as expected).

The CHARGES of the atoms of the same structure calculated with the same method
(AM1) in both programs were also significantly different (for example: the charge in
cabonyl O is ca 0.36 with one program and ca.0.40 with the other). If requested i 
can post the actual outputs. 

I checked the AM1 parameterization references for every atom in both programs 
and it is the same.

Can anybody elucidate this "strange" happenings???

Thanks in advance.

						Mario Tulio Rosado
						Dept of Chemistry
						Faculty of Science
						University  of Lisbon
						PORTUGAL