Subject: CCL:is DFT ab initio?

From: "Oscar N. Ventura" <[email-masked]>
Date: Wed, 17 May 1995 11:31:16 +0200 (MDT)

Dear netters,
 Reading the messages about whether DFT is ab initio or not, I got a 
little bit puzzled. Why is it such a reluctance in admiting DFT as a 
truly separated category of theoretical methods? In my opinion, it is 
clear that semi-empirical and DFT methods share a key characteristic: 
there is no systematic way of improving the results. If for a system you 
got a bad result (be it far from experiment, inconsistent or just crazy) 
using the "best" semi-empirical or DFT methods available, then there is 
no room for systematic improvement. Of course, one can try to 
derive new parametrizations (PM3 vs AM1) or somehow mix calculations and
shake a bit (Becke3LYP vs BLYP), but that is not truly systematic, is it?
 On the other side, DFT and ab initio share the characteristic that they 
are not resorting to experiment to determine the parameters they are 
using (in potentials and in basis sets for instance). In this respect 
they are clearly different from semi-empirical.
 On a second thought, it is also true that semi-empirical and ab initio 
do share some ground in what concerns approximations. What about adding 
-3.07 millihartrees for each bound paired valence electron in Gaussian 1
theory? And is it systematic really the passage from Gaussian 1 to 
Gaussian 2? So then, are Gaussian 1 and Gaussian 2 ab initio theories or 
semi-empirical ones?
 In my opinion, to treat Semi-empirical, Ab initio and DFT methods as 
different approximate models to attempt a solution of the Schroedinger 
equation is the most fair approach. Unless, obviously, that someone may 
think there is more prestige involved in studying DFT if it is catalogued 
as an ab initio theory rather than as a semi-empirical one.
 But surely that is not the case, is it?

	O.
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