Subject: Further "comment on calc. HFS constants"

From: [email-masked] (Ayaz Bakasov, Phys. Chem., ETH Zuerich (Zentrum))
Date: Thu, 2 May 1996 17:11:46 +0200

Dear CCLers,

let me kindly disagree with 
Philippe Maitre <[email-masked]>
and with Steven Creve                       
<[email-masked]>
on the question of "the bad cusp-description using 
gaussians".

The gist of what I want to say
is that the choice of basis
and cusp properties of basis
are PRACTICALLY unrelated things.
So, these are TWO DIFFERENT problems
in computational practice (though a theoretical
purist might still continue his groans :-)   ).

Yes, indeed, Slater-orbitals (STO's)
do have so pleasanty looking cusp at the origin
while Gaussian-orbitals (GTO's) do not.
So what?! If one looks at things closer,
with a pen in hand, then the things appear
to be a bit different from the oversimplified
picture of good STOs and bad GTOs.

The inherent to GTOs lack of cusp at the origin
is not affecting the HFS-constants evaluations 
under consideration, because both the values 
of the atomic orbitals at the origin
and the values of their derivatives at the origin
are entirely defined by the polynomial parts of the
orbitals BUT NOT by their exponential parts.
Take pen and check it!
^^^^^^^^^^^^^^^^^^^^^^

So, when getting unreliable or bizzare results,
take it just as a normal choice-of-basis-set problem
-- the problem one encounters in many many other
applications. It is NOT related to cusp properties
but rather related to bad balancing of electron shells
for the basis set used.

I am myself involved in several calculations
of fine and VERY fine interactions in molecules,
and that's how the cusp-related
things look to me after few years
of experience with them.

Best wishes,
Ayaz Bakasov.