Subject: Questions about SCF convergence again with Gaussian98W and Hyperchem

From: Connie Chang <[email-masked]>
Date: Fri, 08 Nov 2002 13:14:13 -0500

Hi everyone --

thank you for all your good suggestions...

I'm still having trouble getting an optimization and frequency
calculation to work for a charged molecule with 120 atoms.  I've tried
with both Hyperchem and Gaussian98W using the PM3 method.  And I've
tried some fo the suggestions given by listmembers such as increasing
the maximum number of iterations to 500, setting scf=sleazy, setting
scf=qc, etc...  I still get the UNABLE TO ACHIEVE SELF CONSISTENCE error
after (at most) one optimization step.  So the energy will converge
once, it moves the atoms by a little bit for the optimization and the
energy fails to converge.

I have the same problem with hyperchem.  Hyperchem does the neutral case
just fine, but as soon as I add a charge of -1 and a multiplicity of 2,
the energy does not even get close to something reasonable (it's large
and positive).

Does anyone have any further suggestions on what I can do to get either
of the programs to work?  Hyperchem does work for a smaller molecule
(~70 atoms), but Gaussian does not.  Does the size of the molecule make
it more difficult for these programs and methods to find the minimum
energy?  I tried a charged water molecule in Gaussian and it worked
fine....

thanks.
Connie