Subject: [S] G94 Excited States

From: [email-masked] (Ernst U. Wallenborn)
Date: Wed, 29 May 1996 14:06:22 +0200

Hi there,

first of all thanks to all who responded. Below as usual a summary
of the responses. The recommendations included a range of methods
from ZINDO to CASPT2. Several people asked me for a specification 
of the system i'm interested in.

It is all-trans-(1,8)-Diphenyl-octatetraene, C20H16, C2h symmetry. 
(Some people in our group got into Science with a two-photon paper 
recently, that is why my boss wants me to do it).

This is a (20,20) system in g94 nomenclature. Inspection of the CSF
relevant for a simple CIS shows that at least the (10,10) part
is relevant for the first excited singlet and triplet states.

Several people told me to use ZINDO. I don't know how ZINDO would
perform on our system, but would like to try. Unfortunately ZINDO
is commercial. It is available as part of Biosym's insight ii, but
only as an extension, which no insight installation i have access to
features. HyperChem reportedly has ZINDO, too. But then again, this
school here is Spartan territory.

Han Zuilhof <[email-masked]> pointed out that
g94 can do a CISMP2 (CISPT2?) calculation via Iop(9/23=1)
(not 9/21=1 as the manual says). I tried this and quickly found out
that this doesn't work as a direct method. I'll try again and
if it works, i'll let you know.

Then there were the CASPT2 advices. I don't know wether i can
do this for the system of interest. But i'll try.
Besides, is CASSCF MP2 identical to CASSPT2 as i.e. implemented in 
molcas?

Nobody recommended a caculation of the excitation energy via
E[CISDT(e)] -  E[CISD(g)]. (This isn't possible with g94, but
would be in GAMESS (but as i mentioned i don't think my scratch
space will be enough). MOLCAS can do this, too.)
Is there a reason for this? Or is this just a bit out of fashion?

wondering...
--
Ernst-Udo Wallenborn
Laboratorium fuer Physikalische Chemie
ETH Zentrum
CH-8092 Zuerich

Here's the list of messages

The original message was:
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Hi there,

i need to calculate an electronic spectrum of an aromatic compound  
(yes it was my boss' idea, yes i did tell him that this is much
easier to measure and no fun to calculate, no he didn't
find my opinion useful and constructive).

Now the problem is: the thing is far too big for US-GAMESS
(USG has no direct CI, hence it's not expensive to calculate,
it's plain impossible with our HD configuration), it is
too big for a reasonable CAS in g94 (i have a Pi space
of 20 orbitals, of which at least ten have to be included)
CIS on the other hand takes (STO-3g, this was a test)
about 30 minutes on my SPARC 2, but it's results
are, well, not overwhelming.

So the question is: Is there any possibility of doing anything
better than CIS but cheaper than CAS in g94? In GAMESS i'd
do a CISD versus CISD(T) calculation in the style of Shavitt's
70's calculation of the acetone spectrum, but as i mentioned, i think
US-GAMESS is likely to fail. 

I tried 
#3-21g* casscf(10,10,nroot=1) iop(4/17=10,4/18=10,4/19=1)
where the 4/19 option should have set up a FOCI MCSCF (in
GAMESS language), but ended up with a eof error on stdin.
After checking the programmer's manual i added 
   64    1    1
    1    2    3    4    5    1    2    3    4    5    
to the .com, where 64 is the number of occupied orbitals,
and 1 2 3 4 5 1 2 3 4 5 the reference configuration.
That gave me lots of TAPE READ ERRORs.

Besides a test run on water showed a difference between
4/19=2 and GAMESS MCSCF SOCI=.T., so they are not identical 
anyway.

Any help?
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