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

*From*: Connie Chang <cc236(at)cornell.edu>
*Subject*: CCL: Do-it-yourself Freq calculations in Gaussian
*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