Subject: ZINDO ROHF: SOMO below doubly occupied MOs

From: Victor Geskin <[email-masked]>
Date: Tue, 6 May 2003 15:51:31 +0200 (DFT)

Dear CCLers,

Being interested in optical properties of conjugated oligomers, I
calculated electronic structure of oligophenylenevinylene radical cations
with different methods, including ROHF ZINDO.

As the chain length increases, the orbital energy of the SOMO becomes
appreciably lower than that of 1 and then 2 doubly occupied MOs, while in
shorter oligomers SOMO is the highest occupied MO. However, there are all
reasons to believe that the aspect of the SOMO (its symmetry and location
in the middle of the chain) is always correct. Furthermore, the
lowest-energy excitation, as obtained in SCI calculation, is essentially
> from this SOMO to the LUMO.

Of course, in theory MOs must not necessarily be populated by the
electrons in the order of their eigenvalues; it is the total energy that
must be minimum.  However, it is stated in the ZINDO manual that the
aufbau principle is respected for ROHF. In addition, no other method (ab
initio or semiempirical ROHF, unrestricted DFT) makes the SOMO descend
below the doubly occupied MOs.

The question is: is this behavior of ROHF ZINDO correct?

Thanks in advance to everyone who would like to reply.
Victor.
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      Dr Victor GESKIN         e-mail:   [email-masked]

      Service de Chimie                   http://morris.umh.ac.be
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