Subject: CCL: Do-it-yourself Freq calculations in Gaussian

From: Connie Chang <cc236(at)cornell.edu>
Date: Fri, 18 Jul 2003 14:33:14 -0400

Hello members of the CCL list,

I am trying to do a Freq calculation in Gaussian03 with C140 -- two C70 
fullerene cages linked by single bonds.  Whereas the neutral case is 
very pliable and the Opt Freq combined job takes three hours, I am just 
having a devil of a time trying to do the same thing for the charged 
case -- charge = -1.  I have several questions regarding this:

1). The optimization converges, but the freq calculation doesn't get 
beyond the first SCF Energy calculation!  How is this possible since 
many/several SCF energy calculations were already performed during the 
optimization step?

2).  Because my advisor was fed up with the convergence problems of the 
Freq calculation, he told me to calculate the Hessian myself using 
Gaussian and the Force keyword.  So, I optimize the molecule, and then I 
change the coordinates one by one by delta and calculate the Forces that 
result on each atom.  After many jobs (I use a batch file), I reassemble 
my results into a hessian by taking the derivative of the force with 
respect to displacement using a central difference derivative.  I first 
try this with the neutral case since I already have values for the 
frequencies based on my successful Opt Freq run for the neutral 
molecule.  My frequencies don't match up at all -- there are many 
negative eigenvalues for my Hessian despite the fact that my Hessian 
seems to be symmetric.  Am I going about this wrong?  Is there a 
subtlety I am missing?

3).  What things can be tweaked to get the SCF energy to converge?  I've 
already tried SCF=Tight and SCF=QC.  Any other ways people know of?

Thank you and sorry for the long questions...

-Connie