Europium GTH basis set
Florian Schiffmann
flosch... at gmail.com
Sun Dec 22 07:52:31 UTC 2013
Hi Alex,
I recently implented a tool to fit basis sets. So far it seems to work.
Have a look in the tests/QS/regtest-optbas
The idea is that you create a good reference basis set (uncontracted,
expoenents from the ATOM code). With this reference basis you perform
single point calculations on a set of training structures. Usually a good
idea is molecules with different coordination number or the Element with
different charge states.
Afterwards you prepare a basis set with the desired basis set struture,
e.g. DZVP-MOLOPT and run the input according to the example in the tests.
The optimization input contains as well some documentation to describe the
different keywords in a bit more detail.
Two usuful things:
1) sometimes the atomic code creates almost degenerate exponents. This
leads to a very bad condition number and this causes trouble in the
optimization of the final basis.
2) currently I recommend to use the condition number as a fitting criterium
as it becomes dominant at a given point and therefore the important
criteria loose accuracy.
About the method:
The idea is a mixture of the MOLOPT idea and the ADMM method.
It is quite easy to get to the basis set limit with an uncontracted basis
set (usually 6-7 exponents are suficient) and the electronic density
derived with such a basis for different molecules should be optimal. Using
the idea of the auxillary density matrix methods (ADMM) an analytical form
exists to obtain the best match to that density using a different basis.
Using an outer optimzation loop, this can be used to obtain the reduced
basis which gives you the closest representaion of the density obtained at
the basis set limit. Using a set of molecules to fit the densities should
ensure transferability.
Technically the density difference is not the correct criterium to fit
against as the energy should be optimized. Practically this is much faster
and from what I did so far the results are good as well.
Flo
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