Subject: Re: CCL:high-quality GAMESS-US basis set for iodine?

From: val <[email-masked]>
Date: Thu, 30 Dec 1999 13:26:12 +0300

Dear CCL-members! 
  Dear Tony!

> Having reviewed past CCL messages on the subject, I'm aware of the fact
> that the question has been asked before--but no good answer to be
> found...  Is it possible that there are none available?

It seems there is no way  to solve the iodine problem in one 
simple and clear step, at least now. Concerning bases sets, there 
are some  others besides listed in EMSL.

In "Chem.Phys.Lett. 1995, 233, p.249" small (I)n molecules were
studied and the authors suggested new basis set for iodine.

Slightly corrected version of iodine basis set has been used in
"Chem.Phys.Lett. 1993, 202, p.495" to study IHI(-) molecule.

The 6-311G iodine basis set Tony was asking for, with supplementary
diffuse and polarization functions, has been published already, take
a look at "J.Chem.Phys. 1995, 103, p.1878".

A calibration study on HI and I2 molecules has shown that basis set 
with g-functions (25s20p14d4f3g) may be required to get reliable 
results. "Mol.Phys. 1996, 88, p.1645".

Even "the simple" I2 molecule  may require special attention, 
see: "J.Chem.Phys. 1997, 107, p.9046", "J.Phys.Chem. A 1997, 101, 
p.2192".

> What do the professionals use to get good geometries and energies of
> iodine compounds, even as simple as HI?  How much better can MIDI! be
> than 3-21G* in this situation? Is my aversion to ECP justified?  I
> appreciate any comments.

The studies cited above probably indicate that one may forget about 3-21G
or MIDI if one needs something more than rough estimation. For difficult 
cases, like [ICl2]-, one may think on developing a new one ;-)
ECP is not an absolute must, but the cheapest way to take relativistic
effect into account, which can not be neglected for the heaviest of the
halogens.
Moreover, most of the authors suggest CCSD(T) as electron correlation
theory level, which to the best of my knowledge is not implemented in 
GAMESS-US yet. However, you could try MP4 in PC-GAMESS version of 
GAMESS-US instead.

Happy Y2K!

best regards,  Valentin.

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From: [email-masked]
Date: Thu, 30 Dec 1999 12:28:45 +0100 (MET)

Unfortunately, it is not true that ECP account for the "relativistic
effects" in the Iodine molecule:
I guess that spin-orbit effects are more important than scalar-relativistic
effects in this case.
The ECSs implemented e.g. in Gaussian stick to the nonrelativistic
symmetry and do not incorporate spin-orbit effects.

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From: val <[email-masked]>
Date: Thu, 30 Dec 1999 16:15:56 +0300

> symmetry and do not incorporate spin-orbit effects.

Good!

Spin-orbit effect seems to be very important for calculation of various 
properties, e.g. NMR, etc.
Is the same applicable (and really important) for geometry optimization 
as well?

In most cases using ECP usually gives better agreement in geometry 
compared to all-electron calculations for iodine containing molecules. 
For example, one may take a look at recent publications with Stuttgart 
ECP basis set on iodine.

Does anybody know appropriate references on what effect: scalar-
relativistic or spin-orbit has more effect on geometry?

  best regards,   Valentin.

PS. Does "the Iodine molecule" mean I2 only or all iodine containing
species?