Subject: Ab Initio Calcs on Small Pigments

From: [email-masked] (Charles Martin)
Date: Fri, 22 Sep 1995 12:16:03 -0500 (CDT)

Netters,
	
	I am computing the vertical excitation energies of a number
of small pigment molecules (CH2=CH-CH2=NH2+, CH2=CH-CH2=CH-CH2=NH2+, etc,
upto CH2=CH-CH2=CH-CH2=CH-CH2=CH-CH2=CH-CH2=NH2+) using both 
multi-reference third order perturbation theory and simple CASSCF.
I find, however, that the CASSCF tends to underestimate the
excitation energy to the lowest singlet state, compared to the
correlated calculations.  For instance, CASSCF(6-31G*) on 
CH2=CH-CH2=CH-CH2=CH-CH2=CH-CH2=CH-CH2=NH2+, a retinal analog,
gives delta E = 1.898 eV , where MRSDCI with a comparable basis
(by Davidson, JPC 94 (1990)  page 7013) yields 3.6 to 2.7 eV.  

I am simply wondering if anyone else knows of or has had experience 
with similar systems? [I know, for instance, that Roos has studied polyenes 
extensively using the CASSCF and CASSCF + PT2 methods, but not for
protonated polyenes like these which lack Rydberg-Valence mixing.  ]

Also, does anyone know of any good spectroscopic data for cations of
this type?

Thanks in advance

Charles Martin

Beckman Institute
The University of Illinois at Urbana-Champaign

[email-masked]