Subject: Another Gaussian98 question RE Freq and Hessian matrix

From: Connie Chang <[email-masked]>
Date: Wed, 30 Oct 2002 14:42:43 -0500

Hi all --

I'm sorry to be flooding your mailboxes with my silly questions, but
I've discovered another sticking point that I'm not sure how to solve...

I am looking for the Hessian matrix for the molecule I am working with.
The dimensions of the Hessian should be (# of atoms * 3) x (# of atoms
*3)  but I've just discovered that Gaussian only gives
{(# of atoms *3) - 6} normal modes.  The 6 missing are the low-lying
translational/rotational modes I am guessing.  But I need the full
Hessian for the calculation I am considering.  Is the full Hessian
stored elsewhere?  How can I access it?

Thanks,
Connie

---

From: Tom Sundius <[email-masked]>
Date: Wed, 30 Oct 2002 23:26:53 +0200 (EET)

> stored elsewhere?  How can I access it?

I do not think so. For a molecule with 7 atoms, for instance, the number
of Hessian elements is (3*7)^2 = 441, but if you store the lower or upper
triangle, you get 3*7(3*7+1)/2 = 231, which is what I found in the
archive part of the Gaussian output. You are now thinking of the number of
normal modes which is something else.

       Greetings,

        Tom

Tom Sundius
University of Helsinki, Dept of Physical Sciences   phone +358-9-191 50672
P.O.Box 64, FIN-00014 Helsinki, Finland             fax   +358-9-191 50610
+++ for we know in part, and we prophecy in part +++