Subject: AB INITIO vs SEMI-EMP LUMO's

From: "Leo, Howard" <[email-masked]>
Date: Mon, 08 Nov 1993 09:44:13 -0400 (EDT)

Dear Netters,

        I am investigating some chlorinated acroleins
using both semi-empirical and ab initio calculations. While
the HOMO energies from the two methods agree fairly well (~0.5 eV),
I am disturbed by the large differences in the LUMO energies.
(AM1 and PM3 give similar results.)

                 LUMO ENERGIES (OPTIMIZED STRUCTURES)
                  AM1           6-31G           6-31G*
----------------------------------------------------------------------
   C=C
      \         -0.139          +2.280          +2.702
       C=O
----------------------------------------------------------------------
        Cl
       /
    C=C         -0.466          +1.725          +2.231
       \
        C=O
----------------------------------------------------------------------
 Cl
   \
    C=C         -0.873          +1.358          +1.861
   /   \
 Cl     C=O
----------------------------------------------------------------------
 Cl     Cl
   \   /
    C=C         -0.803          +1.396          +1.948
       \
        C=O
---------------------------------------------------------------------

        Can anybody shed some light on this problem? I will
summarize the responses. Thanks.
                                  Howard Leo
                                  LEOH@LEMOYNE