Subject: RE: Deduction of electronic properties from Gaussian output.

From: "Shobe, Dave" <[email-masked]>
Date: Wed, 13 Sep 2000 09:00:30 -0400

> 1. How can I tell if I need to use guess=alter?

I don't know of a way to test a "stable" wavefunction to see if it's the
lowest energy state, anymore than I know of a way to test if a particular
geometry is the lowest energy for the system.  It's the "a local minimum is
not necessarily the global minimum" problem.  You could try switching the
HOMO and LUMO (especially if they have different symmetries) and seeing what
happens.  

One thing I haven't tried but might work: if you do a CIS calculation, maybe
it will show a negative excitation energy if you don't start with a ground
state.  (Reading below, it sounds like you'd like to do a CIS calc. anyway
if you can afford it).

Also, if you're calculating a series of similar complexes, you might see
whether the orbitals (especially the SOMOs) are similar.  You wouldn't
expect NiCl2(NH3)2 and NiCl2(ethylenediamine) to have wildly different
electron configurations for example.

> 2. Is it reasonable to run the same complex as a singlet, triplet,
> quintet, and septet and determine the ground state multiplicity as the
> job that has the lowest energy? 

That's basically what you have to do, although you may be able to rule some
of these out by chemical intuition.  Be careful, though: the HF method
heavily favors high-spin states.  (I.e. there's more correlation energy
within electron pairs than between unpaired electrons).  
I don't know if that applies to B3LYP or not.

> 3. I generally run NBO analysis on these complexes. How do I determine
> the electronic angular momentum?

Can't help with this one.

> 4. Is it reasonable to _estimate_ allowed and forbidden electronic
> transitions by using the orbital energy levels provided from the
> optimized geometries? I understand that CIS and CASSCF calculations
> would provide better results.

I imagine the trends within a series of similar complexes would be useful
information.

> I have been using the UB3LYP/LANL2DZ basis set for the calculations;
> MP2/anything takes too long to converge. 
>
> Roy Jensen

CIS will take even longer. :-(  But at least you only have to do it at a
single point, and if my answer to #4 is correct, you won't have to do it for
every molecule.

--David Shobe
Süd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  [email-masked]

Any opinions herein are not necessarily representative of Süd-Chemie.