[Pw_forum] ab initio vs. first principles.

[David Farrell]

I also had a similar question in my head for a while about the  
ubiquitous use of ab initio == quantum mechanical model in materials  
science and physics literature (nanomechanics as well, for what it's  
worth). Mostly because some of the techniques include things that are  
definitely approximations or involve some sort of parameterization.  
Now, compared to empirical potentials, DFT, Hartree-Fock and even many  
Tight Binding approaches are certainly 'ab initio' because of how and  
where the approximations are made... and this seems to be the  
justification for using 'ab initio' in this manner. Besides, it sounds  
cool when written or said... a reason whose influence probably  
shouldn't be underestimated.

Dave

On Dec 21, 2008, at 7:47 PM, Leonardo Matheus wrote:

> A small doubt that I had while following this discussion, please  
> someone correct-me if I'm wrong:
> Is the following definition right?
>
> "[in ab-initio calculations we] do not make any assumption, such as  
> concerning models and/or fitting parameters."
>
>
> I'm saying this because we use, for example, the homogeneous  
> electron gas model for LDA calculations, nonetheless it is still "ab- 
> initio".
> I think that it's not the use of parameters or models that define a  
> first principles method, but from where those parameters were taken.
> If your parameters came from experimental values, it's somewhat  
> empirical (or semi-empirical at least), if you used just theoretical  
> values (even if using a simplified model) it can be called ab-initio.
>
> Is this right?
>
> This can even create some discussion if some new functionals can be  
> called "ab-initio" or not...
>
> Leonardo Matheus
> IF-USP / Brazil
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David E. Farrell
Post-Doctoral Fellow
Department of Materials Science and Engineering
Northwestern University
email: d-farrell2 at northwestern.edu

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