Subject: Spartan: a multiplicity problem with a porphyrin

From: Hiroshi Ogura <[email-masked]>
Date: Thu, 23 Nov 1995 21:15:08 -0700 (MST)

I am a relatively new user of Spartan.  A while ago, I was attempting an
ab-initio calculation for a diacid form of octamethyl tetraphenyl
porphyrin (H2OMTPP), a nonplanar species.  When the program asked for the
multiplicity, I indicated that it was 1 -- the molecule is diamagnetic. 
Very soon after I submitted the calculation, the program sent me an error
message:  it said that singlet multiplicity was impossible with this
molecule! 

I certainly do know that H2OMTPP is diamagnetic, just like any other
nonmetallated porphyrin.  Moreover, when I submitted an ab-initio
calculation for the diacid form of the simplest porphyrin -- the one with
hydrogens in all of the peripheral positions -- the program did not
complain with my assignment of singlet multiplicity.  How could the
nonplanar distortion of H2OMTPP cause the program to think that the
molecule could not be singlet?  Thanks in advance. 

--
Hiroshi Ogura          | One man, he drinks up his whiskey,
[email-masked]  | Another, he drinks up his wine.
Dept. Chem., Univ. Az. | And they'll drink till their eyes are red with hate
Tucson, AZ 85721       | For those of a different kind. -- Richard Thompson