Dear folks,

                 Several days ago I posted a question regarding the errors
in frequencies calculated using the COSMO model as implemented in Adf
2000.02. The only answer I received is from Cory Pye, who
implement the COSMO model into the ADF package. As a_very_short_summary I
can say that there's no reason to expect additional "noise" introduced by
the solvation model during a frequency calculation.
                 Below is my original posting, Cory's answer follows, and a
final comment is appended by me.
                 Regards,

                                                         Reinaldo

> > Dear Adf'ers,
> >
> > I'm doing some calculations on an organic ligand (30
> atoms) for which
> > solvent (water) effects should be important.
> > Geometry optimizations are ok, very cpu-time consuming
> though. When I run
> > frequency calculations on some conformations of the above ligand,
> including
> > solvent efects as well, one to three imaginary frequencies arise for all
> > the conformations studied. The imaginary frequencies range from 30 to 110
> > cm-1, that is they represent very floppy modes. I distorted one of the
> > above conformations (having one imaginary freq) following the imaginary
> > mode but I dind't succeed, a new imaginary freq arises.
> > Now my question: normal, gas-phase, frequency
> calculations involving
> > numerical second derivatives are subjected to errors of +- 50 cm-1, aren't
> > they? Does the inclusion of the COSMO model affect the error bars for
> > frequencies? That is, is it safe to say that a 100 i cm-1 frequency is an
> > artifact when solvent effects are introduced variationally into the KS scf
> > equations?
> >

>Hello, I am the author of the COSMO routines in ADF.
>The ADF COSMO routine was not tested with the FREQUENCIES option, I assume
>you
>mean Numerical Differentiation of the analytic 1st derivatives and not fully
>analytic frequencies which I believe will be in some future version. There is
>no reason a priori why the KS SCF equations should be affected to any
>degree by
>this, but what I would be more concerned about is the
>creation/annihilation of
>points on the sphere. This could cause the gradients to be a little
>discontinuous. One of the last things I did as a post-doc with Tom Ziegler at
>the UoC is to add a new command called FDIV=X. The default is FD=1 (60 points
>per sphere), you might get better numbers if you tried FD=2 (240 points per
>sphere). I don't know of these changes were incorporated in the latest ADF
>release. This is kind of like the INTEGRATION=X (or was it ACCURACY=X?)
>except
>for surface points.

                 Now it's me again. Adf 2000.02 has a subkey within
SOLVATION named DIV, which controls the technical stuff to construct the
surface. Moreover, it has an option named NDIV, which "controls how fine
the spheres describing the surface are partitioned in small triangles, each
containing one point charge to represent the polarization of the cavity
surface" and has a default value of 3. I guess that these keywords are
equivalent to FDIV and FD, the keywords mentioned by Cory.
Received on 2001-07-11 11:57:34