Subject: Bug in MOPAC 93

From: [email-masked] (Dr. James Stewart)
Date: Thu, 2 Nov 95 09:13:52 PST

BUG IN MOPAC 93

The calculation of excited state transition moments in MOPAC 93 is faulty, and 
results of such calculations should not be used.  The fault can be demonstrated 
by running the following job:

  1scf meci C.I.=2

  H    0.00000000  0    0.0000000  0    0.0000000  0    0    0    0  
  C    1.11816865  1    0.0000000  0    0.0000000  0    1    0    0  
  F    1.33613605  1  109.9107649  1    0.0000000  0    2    1    0  
 Br    1.90383829  1  104.7029401  1  119.6790291  1    2    1    3  
  I    2.05736002  1  107.2678854  1 -126.1994989  1    2    1    3  

This will give the polarization of the transitions as shown below.  Transition 
moments are calculated from state functions using integrals of the type

                           <Phi_i|x|Phi_j>

and these MUST be zero if i=j.  The output indicates that the `transition' from 
State 1 to State 1 has a non-zero polarizability.  Therefore the calculation is 
faulty.  

This bug affects MOPAC 93 only, and appears to be confined to the polarization
part of the electronic excited state calculation.

James J. P. Stewart

  STATE       ENERGY (EV)        Q.N.  SPIN   SYMMETRY     POLARIZATION
         ABSOLUTE    RELATIVE                            X       Y       Z

    1   -0.011305    0.000000     1   SINGLET     A     0.0003  0.0006  0.0019
    2    2.755365    2.766670     2   TRIPLET     A
    3    3.388682    3.399987     3   SINGLET     A     0.0542  0.1115  0.3755
    4    9.202507    9.213812     4   SINGLET     A     0.0025  0.0051  0.0172