3 research outputs found
Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theory
Reliable Prediction of Charge Transfer Excitations in Molecular Complexes Using Time-Dependent Density Functional Theor
Curvature and Frontier Orbital Energies in Density Functional Theory
Perdew et al. discovered two different properties of
exact Kohn–Sham
density functional theory (DFT): (i) The exact total energy versus
particle number is a series of linear segments between integer electron
points. (ii) Across an integer number of electrons, the exchange-correlation
potential “jumps” by a constant, known as the derivative
discontinuity (DD). Here we show analytically that in both the original
and the generalized Kohn–Sham formulation of DFT the two properties
are two sides of the same coin. The absence of a DD dictates deviation
from piecewise linearity, but the latter, appearing as curvature,
can be used to correct for the former, thereby restoring the physical
meaning of orbital energies. A simple correction scheme for any semilocal
and hybrid functional, even Hartree–Fock theory, is shown to
be effective on a set of small molecules, suggesting a practical correction
for the infamous DFT gap problem. We show that optimally tuned range-separated
hybrid functionals can inherently minimize <i>both</i> DD
and curvature, thus requiring no correction, and that this can be
used as a sound theoretical basis for novel tuning strategies
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Probing Ionic Complexes of Ethylene and Acetylene with Vacuum-Ultraviolet Radiation
Mixed
complexes of acetylene–ethylene are studied using
vacuum-ultraviolet (VUV) photoionization mass spectrometry and theoretical
calculations. These complexes are produced and ionized at different
distances from the exit of a continuous nozzle followed by reflectron
time-of-flight mass spectrometry detection. Acetylene, with a higher
ionization energy (11.4 eV) than ethylene (10.6 eV), allows for tuning
the VUV energy and initializing reactions either from a C<sub>2</sub>H<sub>2</sub><sup>+</sup> or a C<sub>2</sub>H<sub>4</sub><sup>+</sup> cation. Pure acetylene and ethylene expansions are separately carried
out to compare, contrast, and hence identify products from the mixed
expansion: these are C<sub>3</sub>H<sub>3</sub><sup>+</sup> (<i>m</i>/<i>z</i> = 39), C<sub>4</sub>H<sub>5</sub><sup>+</sup> (<i>m</i>/<i>z</i> = 53), and C<sub>5</sub>H<sub>5</sub><sup>+</sup> (<i>m</i>/<i>z</i> =
65). Intensity distributions of C<sub>2</sub>H<sub>2</sub>, C<sub>2</sub>H<sub>4</sub>, their dimers and reactions products are plotted
as a function of ionization distance. These distributions suggest
that association mechanisms play a crucial role in product formation
closer to the nozzle. Photoionization efficiency (PIE) curves of the
mixed complexes demonstrate rising edges closer to both ethylene and
acetylene ionization energies. We use density functional theory (ωB97X-V/aug-cc-pVTZ)
to study the structures of the neutral and ionized dimers, calculate
their adiabatic and vertical ionization energies, as well as the energetics
of different isomers on the potential energy surface (PES). Upon ionization,
vibrationally excited clusters can use the extra energy to access
different isomers on the PES. At farther ionization distances from
the nozzle, where the number densities are lower, unimolecular decay
is expected to be the dominant mechanism. We discuss the possible
decay pathways from the different isomers on the PES and examine the
ones that are energetically accessible