# CCL:G: G gaussian calculation with freq=anharm

*From*: Brian Skinn <bskinn---alum.mit.edu>
*Subject*: CCL:G: G gaussian calculation with freq=anharm
*Date*: Wed, 6 Jul 2016 13:48:14 -0400

Pierre,
Assuming you're using the VPT2 method built into G09, a total of 6N-11
Hessian computations are required for a full anharmonic treatment of a
nonlinear system. This would be 205 Hessians for your 36-atom molecule.
The 6N-11 comes about due to the use of a two-sided finite difference
approach to find the derivatives of the force constants with respect to
displacements along each of the normal modes, plus one calculation for the
unperturbed geometry: 2*(3N-6)+1 = 6N-12+1 = 6N-11.
-Brian
On Wed, Jul 6, 2016 at 12:00 PM, Pierre Archirel pierre.archirel*_*u-psud.fr
<owner-chemistry : ccl.net> wrote:
>
> Sent to CCL by: "Pierre Archirel" [pierre.archirel[]u-psud.fr]
> Dear colleagues,
> I am using gaussian 09 RevD01 with freq=anharm on a rather large molecule
> (36 atoms) and the calculation has been now running for 150 hours on an 8
> core machine. I am wondering if the total cpu time can be evaluated. I have
> already got 100 DFT energies, is it possible to know the total number of
> DFT calculations, to be done? Is it related to the 3n-6 degrees of freedom
> of the molecule?
> Many thanks in advnce,
> Pierre Archirel
> LCP, Orsay, France
> pierre.archirel//u-psud.fr>
>
>