127 research outputs found
Bonds, lone pairs, and shells probed by means of on-top dynamical correlations
The Electron Localization Function (ELF) by Becke and Edgecombe [J. Chem.
Phys. {\bf 92}, 5397 (1990)] is routinely adopted as a descriptor of atomic
shells and covalent bonds. Since the ELF and its related quantities find useful
exploitation also in the construction of modern density functionals, the
interest in complementing the ELF is linked to both the quests of improving
electronic structure descriptors and density functional approximations. The ELF
uses information which is available by considering parallel-spin electron pairs
in single-reference many-body states. In this work, we complement this
construction with information obtained by considering antiparallel-spin pairs
whose short-range correlations are modeled by a density functional
approximation. As a result, the approach requires only a contained
computational effort. Applications to a variety of systems show that, in this
way, we gain a spatial description of the bond in H (which is not available
with the ELF) together with some trends not optimally captured by the ELF in
other prototypical situations
An exact Coulomb cutoff technique for supercell calculations
We present a new reciprocal space analytical method to cutoff the long range
interactions in supercell calculations for systems that are infinite and
periodic in 1 or 2 dimensions, extending previous works for finite systems. The
proposed cutoffs are functions in Fourier space, that are used as a
multiplicative factor to screen the bare Coulomb interaction. The functions are
analytic everywhere but in a sub-domain of the Fourier space that depends on
the periodic dimensionality. We show that the divergences that lead to the
non-analytical behaviour can be exactly cancelled when both the ionic and the
Hartree potential are properly screened. This technique is exact, fast, and
very easy to implement in already existing supercell codes. To illustrate the
performance of the new scheme, we apply it to the case of the Coulomb
interaction in systems with reduced periodicity (as one-dimensional chains and
layers). For those test cases we address the impact of the cutoff in different
relevant quantities for ground and excited state properties, namely: the
convergence of the ground state properties, the static polarisability of the
system, the quasiparticle corrections in the GW scheme and in the binding
energy of the excitonic states in the Bethe-Salpeter equation. The results are
very promising.Comment: Submitted to Physical Review B on Dec 23rd 200
Excited-state normal-modes analysis: the case of porphyrins
Excited state normal modes analysis is systematically applied to investigate
and compare relaxation and internal conversion dynamics of a free-base
porphyrin with a novel functional porphyrin derivative. We discuss strenghts
and limitation of the method, and employ it to predict very different dynamical
behaviours in the two compounds and to clarify the role of high reorganization
energy modes in driving the system towards critical regions of the potential
energy landscape. For the functionalized porphyrin, we identify modes of
vibrations along which the energy gap between different excited state potential
energy surfaces within the Q band manifold may vanish, or be significantly
reduced, with respect to the one observed in the bare porphyrin
Exact Coulomb cutoff technique for supercell calculations in two dimensions
We present a reciprocal space technique for the calculation of the Coulomb
integral in two dimensions in systems with reduced periodicity, i.e., finite
systems, or systems that are periodic only in one dimension. The technique
consists in cutting off the long-range part of the interaction by modifying the
expression for the Coulomb operator in reciprocal space. The physical result
amounts in an effective screening of the spurious interactions originated by
the presence of ghost periodic replicas of the system. This work extends a
previous report [C. A. Rozzi et al., Phys. Rev. B 73, 205119 (2006)], where
three-dimensional systems were considered. We show that the use of the cutoffs
dramatically enhances the accuracy of the calculations for a given supercell
size, and it allows to describe two-dimensional systems of reduced periodicity
with substantially less computational effort. In particular, we consider
semiconductor quantum-dot arrays having potential applications in quantum
information technology.Comment: Submitted to PRB 03/03/0
Stability of Dirac cone in artificial graphene
Trabajo presentado al 18th ETSF Workshop celebrado en Luxemburgo del 1 al 4 de Octubre de 2013.ETSF - European Theoretical Spectroscopy Facility I3 (211956).Peer Reviewe
Single-Walled Carbon Nanotubes as Enhancing Substrates for PNA-Based Amperometric Genosensors
A new amperometric sandwich-format genosensor has been implemented on single-walled
carbon nanotubes screen printed electrodes (SWCNT-SPEs) and compared in terms of performance
with analogous genoassays developed using the same methodology on non-nanostructured glassy
carbon platforms (GC-SPE). The working principle of the genosensors is based on the covalent
immobilization of Peptide Nucleic Acid (PNA) capture probes (CP) on the electrode surface,
carried out through the carboxylic functions present on SWCNT-SPEs (carboxylated SWCNT) or
electrochemically induced on GC-SPEs. The sequence of the CP was complementary to a 20-mer
portion of the target DNA; a second biotin-tagged PNA signalling probe (SP), with sequence
complementary to a different contiguous portion of the target DNA, was used to obtain a sandwich
hybrid with an Alkaline Phosphatase-streptavidin conjugate (ALP-Strp). Comparison of the responses
obtained from the SWCNT-SPEs with those produced from the non-nanostructured substrates
evidenced the remarkable enhancement effect given by the nanostructured electrode platforms,
achieved both in terms of loading capability of PNA probes and amplification of the electron transfer
phenomena exploited for the signal transduction, giving rise to more than four-fold higher sensitivity
when using SWCNT-SPEs. The nanostructured substrate allowed to reach limit of detection (LOD)
of 71 pM and limit of quantitation (LOQ) of 256 pM, while the corresponding values obtained with
GC-SPEs were 430 pM and 1.43 nM, respectively
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