[Pw_forum] ab initio vs. first principles.

Leonardo Matheus leommj at usp.br
Mon Dec 22 02:47:17 CET 2008

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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