Subject: CIS optimizations in G92/G94

From: <[email-masked]>
Date: Sun, 4 Feb 1996 14:45:57 -0600

I am doing some excited state optimizations using CIS (CI-singles) 
in Gaussian 92 and 94.  In the section where the CIS energy is 
calculated, the vertical transition energy as well as the most 
important contributing MOs are listed.  For example, when I optimized 
the 2nd excited state for a system:

 CIS WAVEFUNCTION SYMMETRY COULD NOT BE DETERMINED
 Excited State   1:   Singlet-?Sym    5.4132 eV  229.04 nm  f=0.0001
      16 -> 18         -.63415
      16 -> 23          .12012
      16 -> 26          .23947
   
 CIS WAVEFUNCTION SYMMETRY COULD NOT BE DETERMINED
 Excited State   2:   Singlet-?Sym    6.7068 eV  184.86 nm  f=0.0004
      14 -> 17         -.62208
      14 -> 23          .14637
      16 -> 17          .24028
 This State for optimization and/or second-order correction: 
 Total Energy, E(Cis) =  -227.841238416    
 Copying the Cisingles density for this state as the 1-particle RhoCI density.

The optimization proceeded smoothly with the energy monotonically
decreasing, but over the course of the optimization, the character
of the state (in terms of which MOs contributed) changed to:

 CIS WAVEFUNCTION SYMMETRY COULD NOT BE DETERMINED
 Excited State   1:   Singlet-?Sym    6.9558 eV  178.24 nm  f=0.0004
      13 -> 17          .21977
      14 -> 17         -.60807
      14 -> 22          .11198
      14 -> 24         -.10715
      15 -> 17         -.20644
   
 CIS WAVEFUNCTION SYMMETRY COULD NOT BE DETERMINED
 Excited State   2:   Singlet-?Sym    4.3181 eV  287.12 nm  f=0.0012
      16 -> 18          .65217
      16 -> 25         -.12217
      16 -> 26         -.15125
 This State for optimization and/or second-order correction: 
 Total Energy, E(Cis) =  -227.902833782    
 Copying the Cisingles density for this state as the 1-particle RhoCI density.

In many cases higher excited states collapsed to the same state whose
major contibutor is the 16 -> 18 state.  For some states, such a change
in the character of the state was abrupt and signalled that the optimi-
zation had moved onto the wrong excited state surface.  Or else the 
energy suddenly jumped to a higher value, an obvious sign of this.  In 
others the change was smooth, e.g. above, where the 14 -> 17 contribu-
tion gradually decreased, and the 16 -> 18 state increased, indicating 
significant MO mixing.  In some cases when an abrupt change occurred, I 
stopped the calculation at that point and restarted the optimization on 
the root state with mostly 14 -> 17 character above), and the optimi-
zation finished keeping this same MO character.  The resulting geometry 
was consistent with shifts in the electron density in the vertical 
excitation (e.g. excitations out of the pi or into the pi* caused the 
bond to lengthen, excitations into a sigmaCH* led to dissociation of 
the hydrogen).  In other cases attempts to follow the MO character 
failed because repeatedly it would not even get 2 steps before 
collapsing into a state with totally different MO contributions.

My question is:  If I am simulating the process of vertical excitation
followed by relaxation (or dissociation) using the CIS method, is it 
more realistic, i.e. what actually happens experimentally, to follow 
the CIS optimization of a state through to the end even if the contri-
buting MOs change, or to keep the same MO character, and switch to 
optimizing the geometry using another root?  I believe I have been 
using good judgment in my work so far, but would welcome other per-
spectives.

Jay Badenhoop
Department of Chemistry and Biochemistry
Southern Illinois University at Carbondale
[email-masked]