Subject: Re: CCL:collation of responses on ab initio/first principles

From: "Dr. N. SUKUMAR" <[email-masked]>
Date: Wed, 28 Nov 2001 12:59:31 EST

On Tue, 27 Nov 2001 22:23:08 -0800 Eric Scerri wrote:

> 
> Also, I was particularly interested in the posting from Per Ola 
> Norrby who discusses how basis sets are constructed in a 
> semi-empirical manner especially regarding expansion coefficients and 
> contraction.	I would be interested in learning more about this 
> aspect and/or references to where it is discussed in the literature.
> 
> Further comments to me on any of these issues are welcome.
> 

It is my underdstanding that the term "ab initio" simply means that all
atomic/molecular INTEGRALS are computed analytically, without recourse to
empirical parametrization. It DOES NOT imply that the method is exact nor
that the basis set contraction coefficients were obtained without recourse
to parametrization.

See, e.g. Frank L. Pilar, "Elementary Quantum Chemistry" (McGraw Hill, NY,
1968) Chapter 16, page 453.

Even the integrals need NOT be evaluated EXACTLY for a method to be called
ab initio. For instance, Gaussian employs several asymptotic and other
cutoffs to approximate integral evaluation.

The application of the terminology to DFT is justified on the grounds that
while the FUNCTIONALS may be obtained with recourse to parametrization, the
INTEGRALS are evaluated analytically.

Dr. N. Sukumar
Rensselaer Department of Chemistry