Author Topic: User-defined functionals  (Read 11228 times)

lien.bentein

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User-defined functionals
« on: November 07, 2007, 05:25:04 pm »
Hello,

I would like to know if there is an "easy" way to implement functionals as a user? I would like to define a GGA-type functional reported in an article of Grimme, called B97-D, which is not a standard functional in Turbomole.

Can anyone help me with this problem?

Thanks in advance.


idet2

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Re: User-defined functionals
« Reply #1 on: November 10, 2007, 11:57:02 am »
I am interesting in this as well.
If someone knows how, please post the answer.

Regards,

G.

filipp

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Re: User-defined functionals
« Reply #2 on: November 13, 2007, 02:43:53 pm »
Hi,

Thanks for your question about user-defined functionals. I would like to
take the opportunity and comment on why the choice of density functionals
in TURBOMOLE is limited, and why certain functionals are not supported.

Fitting of density functionals to limited sets of empirical data is
much more involved than, e.g., basis set optimization or density
fitting used in RI-type methods. To the best of my knowledge, there is
no guarantee that such fits of a density functional converges to anything
reasonable. Some highly fitted functionals yield accurate atomization
energies, but produce spurious potential energy surfaces [A. D. Boese,
private communication]. Even for  functionals with fewer fitted
parameters such as B97-1, substantial errors (up to 20 pm in bond
lengths) may occur, even within the training sets [R. Ahlrichs,
F. Furche, S. Grimme, Chem. Phys. Lett. 325 (2000), 317].

Density functionals work not because they are empirically fitted, but because
they are based on certain simple models containing important aspects of
exchange and correlation in molecules. These models satisfy a number of
constraints, such as sum rules, scaling relations, bounds, or limiting
behaviors. It is important to understand that functionals
of a given form, e.g., GGAs or meta-GGAs, have an intrinsic accuracy limit
which will show up in a certain scatter of the results for a wide range of
properties and systems. While empirical fits can change the mean value of
the error distribution, they generally do not reduce the scatter [see, e.g,
the discussion at the end of F. Furche, J. P. Perdew, J. Chem. Phys. 124
(2006), 044103]. In other words, such fits do not improve upon the
predictive value, but merely shift errors around.

There has been a great proliferation of empirical density functionals
in recent years. The trend goes to highly specialized functionals fitted for
specific properties and applications, e.g., weakly bonded closed-shell
systems, transition states, hydrogen bonds etc. These functionals do not
contain new physics, but minimize the errors of existing models on
different subsets. While I do not dispute that such functionals might
be useful, I believe that their value is overrated. Many
empirical functionals have had very short lives, and were outdated
before they were released in quantum chemistry programs.

Some users of quantum chemistry codes (not TURBOMOLE users, though) do
not care much about the origin of a result as long as it agrees with
experiment. If the result does not agree, the functional is
modified. I do not see the point of such "calculations". While
some referees still consider results obtained using "designer
functionals" as independent evidence for experimental results, the
number of critics will continue to increase.

Axel Becke realized the limitations of empirical functionals around the
year 2000. Two years before, he had demonstrated that highly fitted
functionals can reach an accuracy of less than 2 kcal/mol mean absolute
error on the extended G2 set [H. L. Schmider, A. D. Becke, J. Chem. Phys.
108 (1998), 9624]. He did not pursue this direction and has been critical
of highly fitted functionals since then: "Ultimately, this is not
satisfactory".

John P. Perdew, arguably an expert in functional development, discourages
the use of empirical functionals: "Strange fits of passion have I known"
[J. P. Perdew, A. Ruzsinszky, J. M. Tao, V. N. Staroverov, G. E. Scuseria,
G. I. Csonka, J. Chem. Phys. 123 (2005), 062201]. He emphasizes that
functionals can, and thus should, be constructed non-empirically, e.g.,
following his "Jacob's Ladder" hierarchy.

As a TURBOMOLE developer I am committed to provide the scientific community
with efficient and stable tools for electronic structure calculations. These
tools should work for a wide range of properties and systems that are important
to chemists. It is a good tradition of TURBOMOLE to offer a limited
choice of carefully selected and maintained options to the user.
The mission of TURBOMOLE is, in the first place, a scientific one,
and it is against our standards to implement, support, and maintain
methods or density functionals whose scientific value is
doubtful.

Best regards,

Filipp Furche
CEO and partner, TURBOMOLE GmbH

lien.bentein

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Re: User-defined functionals
« Reply #3 on: November 14, 2007, 02:03:51 pm »
Hi,

thank you for your quick reply ! It is very interesting to know the TURBOMOLE point-of-view in this matter. And you gave me some useful argumentation and information for further investigation.

Best regards,

Lien Bentein
Ph.D. student Ghent University

XaverW

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Re: User-defined functionals
« Reply #4 on: July 29, 2009, 01:51:40 pm »
Hi, first of all: sorry for bringing up this old thread, but I think it's more convenient than opening a new one on the same topic.

Dear Mr. Furche, I highly share your point of view considering 'specialized functionals'. However, I disagree if you intend to say that's why Turbomole users shouldn't be able to control the functionals they use - because that is missing one important point:
Users DO manipulate the functionals. In Gaussian anyway, but even in Turbomole (as can be seen from e. g. Reiher, Theor. Chem. Acc. 2001, 107, 48)! Hindering other users - the rest of the scientific community - from doing the same simply means to limit the examination of reproducibility, consistence and verification of the other's results. In other words, if you take away this possibility, you do not hinder bad science - but you may hinder its exposure.

In my case, I simply want to check the applicability of the b3lyp* functional from the above literature.... do you really think that should not be possible?

Regards,
Xaver Wurzenberger
Ph. D. student from LMU, Munich

uwe

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Re: User-defined functionals
« Reply #5 on: August 04, 2009, 10:23:53 am »
Hi,

Markus Reiher has not been a Turbomole developer at that time, and he published the results with his own modified version of the source code. But PLEASE do not let us start a discussion about that on the forum - the list of pros and cons is simply too large !!!

It is however possible in the current Turbomole version to change the amount of HF exchange in the B3-LYP functional. This is done by a non-documented keyword. Please contact the Turbomole support from COSMOlogic to get a short instruction how to perform such calculations.

Regards,

Uwe