Subject: MOPAC; Degenerate Energy levels....

From: [email-masked] (Jason Lye)
Date: Thu, 16 Nov 95 16:46:04 -0500

Dear All,

A couple of weeks ago, I asked the list for some advice concerning an error 
message that was plaguing my work.  

Many thanks to the following people for responding:

Ernest Chamot
Yong Huang
Jimmy Stewart
Gerd Thys

The error message is:

`Degenerate energy levels detected in the MECI.  Some of these levels would be 
treated by MECI, while others would not.  This would lead to non-reproducable 
electronic energies.
Job stopped;  to continue specify GEO-OK."

I tried to use the keyword PRECISE to try and resolve the energy levels, but 
this did not work.  In the end, I used GEO-OK to prevent the prevent the 
program from exiting.

The first part of my question was:
What does this error message mean?

The M.O. energy levels at the end of the SCF calculation are checked to see if
a C.I. can be done in a valid manner.  Orbitals with energies within 0.1eV are
considered degenerate by MOPAC.  If the active space of the C.I. includes some
but not all of the degenerate orbitals, then the C.I. is not meaningful.

What are the consequences of specifying GEO-OK?

The consequences of specifying GEO-OK are to make the calculation meaningless 
in the case of SCF calculations.  In the case of geometry searches and reaction 
pathways, then the keyword may be used, as the geometry and orbital energies
will change.  The calculation may be tested once a stable geometry is obtained 
by running an SFC CI calculation - If no error message is generated, then it's 
fine.

Things to do to try and avoid generation of the error message:

i)	PRECISE  -  Does not prevent the error from arising
ii)	Use a different C.I. level to try and exclude all the degenerate M.O.s
	or including all of them
iii)	Specify the C.I. Manifold more explicitly.  Use, eg, C.I.=(5,3)

Thank you all for your help, and I hope that this summary is helpful to some
of you,

Jason
_______________________________________________________________________________

Jason Lye,                       |    
Dye Synthesis Research Group,    |    
College Of Textiles,  Box 8301,  |     "If we did'nt have any bones, 
North Carolina State University, |      we would have to live in a bucket."
Raleigh, N.C. 27695 - 8301       |     
                                 |      
      Ph:   (919) 515-6615       |                     
      [email-masked]           |         
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