[Wien] Ab inition and DFT

[pboulet]

Hello,

Maybe the difference you are referring to is that between what theoretical chemist consider as "ab initio" and what physicists do.

This was a matter of controversy in the 1990’s, and to me, is more philosophical that physical.

For theoretical chemist, ab initio methods are methods based on the wavefunction, namely Hartee-Fock, and post-Hartree-Fock techniques (e.g., CAS-MCSCF, coupled cluster, CISD, Full-CI). In this sense, DFT is not ab initio as the main ingredient of DFT is the electron density, which is obtained from an approximate description of XC interelectronic interactions.

For physicists DFT is ab initio as the method is grounded on a theorem (HK one) and "starts from scratch".

Final note: for Walter Kohn, DFT is ab initio, not a semi-empirical methods (as many people coming from the community of theoretical chemists used to claim in the 1990’s), but its usage (LDA, GGA,…) is "something else » (if I remember well he said "it is just physics », [of interelectronic interactions]), as it relies on approximations in the XC functionals.

Today, this controversy has been shelved, and everybody, as far as I could see, use indiscernibly DFT or ab initio.

Best,
Pascal


> Le 7 juil. 2022 à 13:59, Fecher, Gerhard <fecher at uni-mainz.de> a écrit :
> 
> This is an unsuitable question, you ask what is the difference between
> "from the beginning" and "density functional theory"
> 
> probably one can speculate whether you might mean
>   " What is the difference between "Hartre Fock", "Hohenberg Kohn", and "Kohn Sham" theoretical approaches to the elecstronic structure of atoms, molecules, or solids?"
> In that case, a good textbook  or the original works might help you the latter rather better than second hand multiple genealogy references ==> do your study ab initio !
> 
> Ciao
> Gerhard
> 
> DEEP THOUGHT in D. Adams; Hitchhikers Guide to the Galaxy:
> "I think the problem, to be quite honest with you,
> is that you have never actually known what the question is."
> 
> ====================================
> Dr. Gerhard H. Fecher
> Institut of Physics
> Johannes Gutenberg - University
> 55099 Mainz
> ________________________________________
> Von: Wien [wien-bounces at zeus.theochem.tuwien.ac.at] im Auftrag von ben amara imen [imen.benamara5 at gmail.com]
> Gesendet: Mittwoch, 6. Juli 2022 15:18
> An: wien at theochem.tuwien.ac.at; wien at zeus.theochem.tuwien.ac.at
> Betreff: [Wien] Ab inition and DFT
> 
> Dears users
> 
> Can someone please  explain the difference between ab initio calculation and DFT.
> 
> As I know , ab initio calculation is based on wave function without any empirical parameters . DFT is based on density of charge but it uses the empirical parameters (within Exchange-correlation potentiel PBE , GGA..).
> 
> Is it correct ?.
> 
> If it is correct , why some papers consider DFT as ab initio calculation.
> 
> Thanks
> 
> 
> Best regards
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Pascal Boulet
—
Professor in computational materials chemistry - DEPARTMENT OF CHEMISTRY
University of Aix-Marseille - Avenue Escadrille Normandie Niemen - F-13013 Marseille - FRANCE
Tél: +33(0)4 13 55 18 10 - Fax : +33(0)4 13 55 18 50
Email : pascal.boulet at univ-amu.fr <mailto:pascal.boulet at univ-amu.fr>




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