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["Anglade Pierre-Matthieu" <anglade@gmail.com>]

Hi, let me try some definitions :

First principle : This english _expression_, used in the context of electronic structure calculations, describes the fact that the simulations are grounded on the very first principles of physics. In this context, this is essentially the Schrödinger equation. Frist principle doesn't mean that this equation is exactly solved. It means that the calculations gives an approximation to the solution that is not grounded on experiment but on theory.

<i>Ab initio</i> : This latin locution, used in the context of electronic structure calculation, describes the
fact that the simulations are going from (~<i>ab</i>) the initial (~<i>initio</i>) theory --- in this context, this is essentially the Schrödinger equation --- to the simulation of matter properties.
<i>Ab initio</i> doesn't mean that Shrödinger's equation is exactly solved. It
means that the calculations gives an approximation to the solution
that is not grounded on experiment but on theory.

Etymology doesn't make a wide difference between those two _expression_. It is not impossible that one or another community prefers using one of this term for DFT and the other for all kinds of quantum calculations ; or conversely reserves one of those phrasal for methods whit less approximations than DFT. Yet I don't believe, that the later distinctions  are strongly grounded way of _expression_, and that there is a widespread consensus on a clear distinction between <i>"ab initio"</i> and "first principles". Therfore I would encourage you to use both, according to your taste. 

regards

PMA

On Sun, Dec 21, 2008 at 12:46 PM, Hongsheng Zhao <zhaohscas@yahoo.com.cn> wrote:

Hi all,

Some times we say ab initio, sometimes first principles.  Actually,
I find the difference of these two phrases are often the same meanings in many papers.  But the fact may be not the case, who can give some hints on the differences between ab initio and first principles?

Regards,
--
Hongsheng Zhao <zhaohscas@yahoo.com.cn>
Xinjiang Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
GnuPG DSA: 0xD108493
2008-12-21

-- 
Pierre-Matthieu Anglade