[CPMD-list] Lame question about functionals.

[Axel Kohlmeyer]

On Mon, 30 May 2005, Jacek Wojdel wrote:

jacek,

there are two answers to your question.

JW> 	Dear CPMD users (and authors),
JW> My simple question is as follows.
JW> If I want to use PW91 GGA functional for both exchange and
JW> correlation, is the following enough:

the recommended way to select this functional is:

FUNCTIONAL GGA

there is no mentioning of a PW91 keyword in the manual
or in the sourcecode (dftin.F). so for any of your options
you should get a warning.

JW> &DFT
JW>  FUNCTIONAL PW91
JW>  GC-CUTOFF
JW>   1.D-5
JW> &END
JW> 
JW> Or should I rather put:
JW> 
JW> &DFT
JW>  GRADIENT CORRECTION PW91 PW91
JW>  GC-CUTOFF
JW>   1.D-5
JW> &END

JW> And if the latter is correct, what is the meaning of the first one? Is
JW> it then LDA calculation with "FUNCTIONAL PW91" line ignored by CPMD?
JW> The manual seems to be rather ambiguous here, so any help will be
JW> appreciated.

well, this is mostly a legacy feature. when using the
OLDCODE feature, the LDA exchange and correlation, as well as
GGA contribution are calculated separately and could thus also
be mixed and matched (almost) at will. there are still some
options, where you have to use this (less efficient than
with NEWCODE) way of computing the DFT part.

if you don't want to mess with this, you just use
FUNCTIONAL XXX and you will get all the parameters 
set properly for the LDA and GGA parts. in some cases,
e.g. PBE, there are small differences, as in combination with
NEWCODE a specially optimized implementation is used, where
the corresponding LDA part is integrated and not 
available with the old style selection mechanism.

please have a look at dftin.F if you want to find out
what those flags are really doing. the part in the manual, 
that you have to look into the sourcecode, if you want to
find out the details, is not a joke.

also, if you think that there is a different, more lucid
way of documenting this, please feel free to submit 
an updated text.

regards,
	axel kohlmeyer.


JW> Regards,
JW> 	Jacek Wojdel
JW> 

-- 

=======================================================================
Dr. Axel Kohlmeyer   e-mail: axel.kohlmeyer at theochem.ruhr-uni-bochum.de
Lehrstuhl fuer Theoretische Chemie          Phone: ++49 (0)234/32-26673
Ruhr-Universitaet Bochum - NC 03/53         Fax:   ++49 (0)234/32-14045
D-44780 Bochum  http://www.theochem.ruhr-uni-bochum.de/~axel.kohlmeyer/
=======================================================================
If you make something idiot-proof, the universe creates a better idiot.