4,664 research outputs found
Variation of parameters in Becke‐3 hybrid exchange‐correlation functional
We have investigated the consequences of varying the three parameters in Becke's hybrid exchange‐correlation functional, which includes five contributions: Hartree–Fock exchange, local exchange, Becke's gradient exchange correction, local correlation, and some form of gradient correlation correction. Our primary focus was upon obtaining orbital energies with magnitudes that are reasonable approximations to the electronic ionization potentials; however, we also looked at the effects on molecular geometries and atomization enthalpies. A total of 12 parameter combinations was considered for each of three different gradient correlation corrections: the Lee–Yang–Parr, the Perdew‐86, and the Perdew–Wang 91. Five molecules were included in the study: HCN, N2, N2O, F2O, and H2O. For comparison, a Hartree–Fock calculation was also carried out for each of these. The 6‐31+G** basis set was used
Development of Novel Density Functionals for Thermochemical Kinetics
A new density functional theory (DFT) exchange-correlation functional for the
exploration of reaction mechanisms is proposed. This new functional, denoted
BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for
transition state barriers but, unlike previous attempts at such a functional,
this improved accuracy does not come at the expense of equilibrium properties.
This makes it a general-purpose functional whose domain of applicability has
been extended to transition states, rather than a specialized functional for
kinetics. The improvement in BMK rests on the inclusion of the kinetic energy
density together with a large value of the exact exchange mixing coefficient.
For this functional, the kinetic energy density appears to correct `back' the
excess exact exchange mixing for ground-state properties, possibly simulating
variable exchange.Comment: J. Chem. Phys., in press (303431JCP, scheduled for August 15, 2004
issue); supplementary data available at
http://theochem.weizmann.ac.il/web/papers/BMK.htm
On how good DFT exchange-correlation functionals are for H bonds in small water clusters: Benchmarks approaching the complete basis set limit
The ability of several density-functional theory (DFT) exchange-correlation
functionals to describe hydrogen bonds in small water clusters (dimer to
pentamer) in their global minimum energy structures is evaluated with reference
to second order Moeller Plesset perturbation theory (MP2). Errors from basis
set incompleteness have been minimized in both the MP2 reference data and the
DFT calculations, thus enabling a consistent systematic evaluation of the true
performance of the tested functionals. Among all the functionals considered,
the hybrid X3LYP and PBE0 functionals offer the best performance and among the
non-hybrid GGA functionals mPWLYP and PBE1W perform the best. The popular BLYP
and B3LYP functionals consistently underbind and PBE and PW91 display rather
variable performance with cluster size.Comment: 9 pages including 4 figures; related publications can be found at
http://www.fhi-berlin.mpg.de/th/th.htm
Accurate Band Gaps for Semiconductors from Density Functional Theory
An essential issue in developing semiconductor devices for photovoltaics and thermoelectrics is to design materials with appropriate band gaps plus the proper positioning of dopant levels relative to the bands. Local density (LDA)
and generalized gradient approximation (GGA) density functionals generally underestimate band gaps for semiconductors and sometimes incorrectly predict
a metal. Hybrid functionals that include some exact Hartree-Fock exchange are known to be better. We show here for CuInSe_2, the parent compound of the promising CIGS Cu(In_xGa_(1-x))Se_2 solar devices, that LDA and GGA obtain gaps of 0.0-0.01 eV (experiment is 1.04 eV), while the historically first global hybrid functional, B3PW91, is surprisingly better than B3LYP with band gaps of 1.07 and
0.95 eV, respectively. Furthermore, we show that for 27 related binary and ternary semiconductors, B3PW91 predicts gaps with a mean average deviation (MAD) of only 0.09 eV, which is substantially better than all modern hybrid functionals
Extent of Fock-exchange mixing for a hybrid van der Waals density functional?
The vdW-DF-cx0 exchange-correlation hybrid design has a truly nonlocal
correlation component and aims to facilitate concurrent descriptions of both
covalent and non-covalent molecular interactions. The vdW-DF-cx0 design mixes a
fixed ratio, , of Fock exchange into the consistent-exchange van der Waals
density functional, vdW-DF-cx. The mixing value is sometimes taken as a
semi-empirical parameter in hybrid formulations. Here, instead, we assert a
plausible optimum average value for the vdW-DF-cx0 design from a formal
analysis; A new, independent determination of the mixing is necessary since
the Becke fit, yielding , is restricted to semilocal correlation and
does not reflect non-covalent interactions. To proceed, we adapt the so-called
two-legged hybrid construction to a starting point in the vdW-DF-cx functional.
For our approach, termed vdW-DF-tlh, we estimate the properties of the
adiabatic-connection specification of the exact exchange-correlation
functional, by combining calculations of the Fock exchange and of the
coupling-constant variation in vdW-DF-cx. We find that such vdW-DF-tlh hybrid
constructions yield accurate characterizations of molecular. The accuracy
motivates trust in the vdW-DF-tlh determination of system-specific values of
the Fock-exchange mixing. We find that an average value best
characterizes the vdW-DF-tlh description of covalent and non-covalent
interactions, although there exists some scatter. This finding suggests that
the original Becke value, , also represents an optimal average
Fock-exchange mixing for the new, truly nonlocal-correlation hybrids. To enable
self-consistent calculations, we furthermore define and test a zero-parameter
hybrid functional vdW-DF-cx0p (having fixed mixing ) and document that
this truly nonlocal correlation hybrid works for general molecular
interactions.Comment: 18 pages, 5 figures, accepted by J. Chem. Phy
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