A new density functional (DF) of the generalized gradient approximation (GGA) type
for general chemistry applications termed B97-D is proposed. It is based on Becke's
power-series ansatz from 1997 and is explicitly parameterized by including damped
atom-pairwise dispersion corrections of the form C(6) x R(-6). A general computational
scheme for the parameters used in this correction has been established and parameters
for elements up to xenon and a scaling factor for the dispersion part for several
common density functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional
is tested in comparison with other GGAs and the B3LYP hybrid functional on standard
thermochemical benchmark sets, for 40 noncovalently bound complexes, including large
stacked aromatic molecules and group II element clusters, and for the computation
of molecular geometries. Further cross-validation tests were performed for organometallic
reactions and other difficult problems for standard functionals. In summary, it is
found that B97-D belongs to one of the most accurate general purpose GGAs, reaching,
for example for the G97/2 set of heat of formations, a mean absolute deviation of
only 3.8 kcal mol(-1). The performance for noncovalently bound systems including many
pure van der Waals complexes is exceptionally good, reaching on the average CCSD(T)
accuracy. The basic strategy in the development to restrict the density functional
description to shorter electron correlation lengths scales and to describe situations
with medium to large interatomic distances by damped C(6) x R(-6) terms seems to be
very successful, as demonstrated for some notoriously difficult reactions. As an example,
for the isomerization of larger branched to linear alkanes, B97-D is the only DF available
that yields the right sign for the energy difference. From a practical point of view,
the new functional seems to be quite robust and it is thus suggested as an efficient
and accurate quantum chemical method for large systems where dispersion forces are
of general importance.
Copyright 2006 Wiley Periodicals, Inc.