246 research outputs found
A Lanczos algorithm for linear response
An iterative algorithm is presented for solving the RPA equations of linear
response. The method optimally computes the energy-weighted moments of the
strength function, allowing one to match the computational effort to the
intrinsic accuracy of the basic mean-field approximation, avoiding the problem
of solving very large matrices. For local interactions, the computational
effort for the method scales with the number of particles N_p as O(N_p^3).Comment: 12 pages including 3 figures; Late
Oscillator strengths with pseudopotentials
The time-dependent local-density approximation (TDLDA) is shown to remain
accurate in describing the atomic response of IB elements under the additional
approximation of using pseudopotentials to treat the effects of core electrons.
This extends the work of Zangwill and Soven who showed the utility of the
all-electron TDLDA in the atomic response problem.Comment: 13 pages including 3 Postscript figure
Application of time-dependent density functional theory to optical activity
As part of a general study of the time-dependent local density approximation
(TDLDA), we here report calculations of optical activity of chiral molecules.
The theory automatically satisfies sum rules and the Kramers-Kronig relation
between circular dichroism and optical rotatory power. We find that the theory
describes the measured circular dichroism of the lowest states in methyloxirane
with an accuracy of about a factor of two. In the chiral fullerene C_76 the
TDLDA provides a consistent description of the optical absorption spectrum, the
circular dichroism spectrum, and the optical rotatory power, except for an
overall shift of the theoretical spectrum.Comment: 17 pages and 13 PostScript figure
Wavelet-Based Linear-Response Time-Dependent Density-Functional Theory
Linear-response time-dependent (TD) density-functional theory (DFT) has been
implemented in the pseudopotential wavelet-based electronic structure program
BigDFT and results are compared against those obtained with the all-electron
Gaussian-type orbital program deMon2k for the calculation of electronic
absorption spectra of N2 using the TD local density approximation (LDA). The
two programs give comparable excitation energies and absorption spectra once
suitably extensive basis sets are used. Convergence of LDA density orbitals and
orbital energies to the basis-set limit is significantly faster for BigDFT than
for deMon2k. However the number of virtual orbitals used in TD-DFT calculations
is a parameter in BigDFT, while all virtual orbitals are included in TD-DFT
calculations in deMon2k. As a reality check, we report the x-ray crystal
structure and the measured and calculated absorption spectrum (excitation
energies and oscillator strengths) of the small organic molecule
N-cyclohexyl-2-(4-methoxyphenyl)imidazo[1,2-a]pyridin-3-amine
Efficient Benzodithiophene/Benzothiadiazole-Based n-Channel Charge Transporters
A series of donorâacceptor small molecules based on electrondeficient benzothiadiazole (BTD) and electron-rich benzodithiophene (BDT) featuring an A-D-A structure is presented. Exhaustive spectroscopic, electrochemical, and computational studies evidence their electroactive nature and their ability to form well-ordered thin films with broad visible absorptions and low band gaps (ca. 2 eV). Time-resolved microwave conductivity (TRMC) studies unveil unexpected n-type charge transport displaying high electron mobilities around 0.1 cm2V1 s 1 . Efficient electron transport properties are consistent with the low electron reorganization energies (0.11â0.17 eV) theoretically predicted
Quasiparticle Density-Matrix Representation of Nonlinear TDDFT Response Functions
The time-dependent density functional (TDDFT) equations may be written either
for the Kohn-Sham orbitals in Hilbert space or for the single electron density
matrix in Liouville space. A collective-oscillator, quasiparticle,
representation of the density response of many-electron systems which
explicitly reveals the relevant electronic coherence sizes is developed using
the Liouville space representation of adiabatic TDDFT. Closed expressions for
the nonlinear density-density response are derived, eliminating the need to
solve nonlinear integral equations, as required in the Hilbert space
formulation of the response.Comment: 24 page
Three real-space discretization techniques in electronic structure calculations
A characteristic feature of the state-of-the-art of real-space methods in
electronic structure calculations is the diversity of the techniques used in
the discretization of the relevant partial differential equations. In this
context, the main approaches include finite-difference methods, various types
of finite-elements and wavelets. This paper reports on the results of several
code development projects that approach problems related to the electronic
structure using these three different discretization methods. We review the
ideas behind these methods, give examples of their applications, and discuss
their similarities and differences.Comment: 39 pages, 10 figures, accepted to a special issue of "physica status
solidi (b) - basic solid state physics" devoted to the CECAM workshop "State
of the art developments and perspectives of real-space electronic structure
techniques in condensed matter and molecular physics". v2: Minor stylistic
and typographical changes, partly inspired by referee comment
Transition-metal dimers and physical limits on magnetic anisotropy
Recent advances in nanoscience have raised interest in the minimum bit size
required for classical information storage, i.e. for bistability with
suppressed quantum tunnelling and energy barriers that exceed ambient
temperatures. In the case of magnetic information storage much attention has
centred on molecular magnets[1] with bits consisting of ~ 100 atoms, magnetic
uniaxial anisotropy energy barriers ~ 50 K, and very slow relaxation at low
temperatures. In this article we draw attention to the remarkable magnetic
properties of some transition metal dimers which have energy barriers
approaching ~ 500 K with only two atoms. The spin dynamics of these ultra small
nanomagnets is strongly affected by a Berry phase which arises from
quasi-degeneracies at the electronic Highest Occupied Molecular Orbital (HOMO)
energy. In a giant spin-approximation, this Berry phase makes the effective
reversal barrier thicker. [1] Gatteschi, D., Sessoli, R. & Villain, J.
Molecular Nanomagnets. (Oxford, New York 2006).Comment: 14 pages, 1 figur
Neutral and Oxidized Triisopropylsilyl End-Capped Oligothienoacenes: A Combined Electrochemical, Spectroscopic, and Theoretical Study
This work presents an analysis of the structural, electrochemical, and optical properties of a family of triisopropylsilyl end-capped oligothienoacenes (TIPS- T n -TIPS, n =4â8) by combining cyclic voltammetry, spectroscopic techniques, and quantum-chemical calculations. TIPS- T n -TIPS compounds form stable radical cations, and dications are only obtained for the longest oligomers ( n =7 and 8). Oxidation leads to the quinoidization of the conjugated backbone, from which electrons are mainly extracted. The absorption and fluorescence spectra show partially resolved vibronic structures even at room temperature, due to the rigid molecular geometry. Two well-resolved vibronic progressions are observed at low temperatures due to the vibronic coupling, with normal modes showing wavenumbers of â1525 and â480â14cm â1 . Optical absorption bands display remarkable bathochromic dispersion with the oligomer length, indicative of the extent of Î conjugation. The optical properties of the oxidized compounds are characterized by in situ UV/Vis/NIR spectroelectrochemistry. The radical cation species show two intense absorption bands emerging at energies lower than in the neutral compounds. The formation of the dication is only detected for the heptamer and the octamer, and shows a new band at intermediate energies. Optical data are interpreted with the help of density functional theory calculations performed at the B3LYP/6-31G** level, both for the neutral and the oxidized compounds.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/75765/1/5481_ftp.pd
A well-tempered density functional theory of electrons in molecules
Reporting extensions of a recently developed approach to density functional
theory with correct long-range be-havior (Phys. Rev. Lett. 94, 043002 (2005)).
The central quantities are a splitting functional gamma[n] and a complementary
exchange-correlation functional. We give a practical method for determining the
value of \gamma in molecules, assuming an approximation for XC energy is given.
The resulting theory shows good ability to reproduce the ionization potentials
for various molecules. However it is not of sufficient accuracy for forming a
satisfactory framework for studying molecular properties. A somewhat different
approach is then adopted, which depends on a density-independent \gamma and an
additional parameter w eliminating part of the local exchange functional. The
values of these two parameters are obtained by best-fitting to experimental
atomization energies and bond-lengths of the molecules in the G2(1) database.
The optimized values are gamma=0.5 a_0^{-1} and w=0.1 . We then examine the
performance of this slightly semi-empirical functional for a variety of
molecular properties, comparing to related works and to experiment. We show
that this approach can be used for describing in a satisfactory manner a broad
range of molecular properties, be they static or dynamic. Most satisfactory is
the ability to describe valence, Rydberg and inter-molecular charge-transfer
excitations
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