Subject: Re: CCL:G:G94: B3LYP energy and Charge

From: [email-masked] (Ross Nobes)
Date: Wed, 6 Dec 1995 10:33:05 +0000 (GMT)

Mike Frisch writes:
  > 
  > > The answer is that both are right!  The Gaussian definition of B3LYP
  > > uses the VWN local correlation functional;  most other programs use
  > > what Gaussian terms VWN5.
  > > 
  > 
  > No, B3LYP was defined to use VWN3.  Using the same name for two different
  > functionals is a disservice to everyone and makes reproducing other peoples
  > results impossible.  This used to be a major problem in the DFT area, and
  > things are finally improving with regard to terminology.  To take a name for
  > something that is specified in the literature and use it for something
  > different is a major step backwards.  Published and in-press tests of B3LYP
  > all use VWN3; an assumption that the same conclusions apply to a different
  > functional just because it has the same name are not valid.
  > 

Point taken.  All that I was implying in my original answer was that there
are no 'numerical' problems here - both programs are giving correct results
within their own definitions of B3LYP.  Most programs (such as Mulliken,
CADPAC, MOLPRO, Turbomole) use the VWN5 functional uniformly as the 
local correlation part of functionals such as SVWN, B3P86 and B3LYP.  It
is unfortunate that Gaussian is alone in using an alternative VWN local
correlation functional and (as Mike says) that the terminology is not 
consistently defined.  The issue is further confused by a bug in the
Gaussian 92/DFT open-shell VWN5 code which leads to small discrepancies
with the other programs even when extremely large quadrature grids are
used.

-- 
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| Dr Ross Nobes                                                  |
| Senior Scientist, Quantum Mechanics                            !
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From: [email-masked] (Mike Frisch)
Date: Wed, 6 Dec 1995 09:24:00 -0500 (EST)

> with the other programs even when extremely large quadrature grids are

This bug, long since fixed, is not relevent to the question of using terms
consistently.  BLYP, BP, etc. are consistent names for functionals.  This is
a big improvement over "LSDA" which can mean many different things -- not
just VWN3 vs. VWN5 but Perdew 86, the local part of Perdew 91, and other
local correlation functionals as well can be found referred to as LSDA in
the literature.  The confusion over LSDA should not be used to rationalize
adding to the confusion with new functionals.  Things improved more recently
in that, for example, everyone means the same thing by BLYP or BP (=BPW91).
B3LYP numbers were available in the literature for comparison and there
really isn't any excuse for doing something else and claiming ignorance that
the results are different.  The comparisons that Ross mentions having made
between codes for other functionals can be made by anyone implementing
B3LYP, or made using literature results.

Mike Frisch
[email-masked]