3,019 research outputs found
Accurate Treatment of Large Supramolecular Complexes by Double-Hybrid Density Functionals Coupled with Nonlocal van der Waals Corrections
In this work, we present a thorough assessment of the performance of some representative double-hybrid density functionals (revPBE0-DH-NL and B2PLYP-NL) as well as their parent hybrid and GGA counterparts, in combination with the most modern version of the nonlocal (NL) van der Waals correction to describe very large weakly interacting molecular systems dominated by noncovalent interactions. Prior to the assessment, an accurate and homogeneous set of reference interaction energies was computed for the supramolecular complexes constituting the L7 and S12L data sets by using the novel, precise, and efficient DLPNO-CCSD(T) method at the complete basis set limit (CBS). The correction of the basis set superposition error and the inclusion of the deformation energies (for the S12L set) have been crucial for obtaining precise DLPNO-CCSD(T)/CBS interaction energies. Among the density functionals evaluated, the double-hybrid revPBE0-DH-NL and B2PLYP-NL with the three-body dispersion correction provide remarkably accurate association energies very close to the chemical accuracy. Overall, the NL van der Waals approach combined with proper density functionals can be seen as an accurate and affordable computational tool for the modeling of large weakly bonded supramolecular systems.Financial support by the âMinisterio de EconomĂa y Competitividadâ (MINECO) of Spain and European FEDER funds through projects CTQ2011-27253 and CTQ2012-31914 is acknowledged. The support of the Generalitat Valenciana (Prometeo/2012/053) is also acknowledged. J.A. thanks the EU for the FP7-PEOPLE-2012-IEF-329513 grant. J.C. acknowledges the âMinisterio de EducaciĂłn, Cultura y Deporteâ (MECD) of Spain for a predoctoral FPU grant
OBLIQUE PROJECTION METHODS FOR LARGE-SCALE MODEL-REDUCTION
Published versio
Performance of ab initio and density functional methods for conformational equilibria of CnH2n+2 alkane isomers (n=2-8)
Conformational energies of n-butane, n-pentane, and n-hexane have been
calculated at the CCSD(T) level and at or near the basis set limit.
Post-CCSD(T) contribution were considered and found to be unimportant. The data
thus obtained were used to assess the performance of a variety of density
functional methods. Double-hybrid functionals like B2GP-PLYP and B2K-PLYP,
especially with a small Grimme-type empirical dispersion correction, are
capable of rendering conformational energies of CCSD(T) quality. These were
then used as a `secondary standard' for a larger sample of alkanes, including
isopentane and the branched hexanes as well as key isomers of heptane and
octane. Popular DFT functionals like B3LYP, B3PW91, BLYP, PBE, and PBE0 tend to
overestimate conformer energies without dispersion correction, while the M06
family severely underestimates GG interaction energies. Grimme-type dispersion
corrections for these overcorrect and lead to qualitatively wrong conformer
orderings. All of these functionals also exhibit deficiencies in the conformer
geometries, particularly the backbone torsion angles. The PW6B95 and, to a
lesser extent, BMK functionals are relatively free of these deficiencies.
Performance of these methods is further investigated to derive conformer
ensemble corrections to the enthalpy function, , and the Gibbs
energy function, , of these alkanes. While
is only moderately sensitive to the level of theory, exhibits more pronounced sensitivity. Once again, double hybrids
acquit themselves very well.Comment: J. Phys. Chem. A, revised [Walter Thiel festschrift
Elastic and vibrational properties of alpha and beta-PbO
The structure, electronic and dynamic properties of the two layered alpha
(litharge) and beta (massicot) phases of PbO have been studied by density
functional methods. The role of London dispersion interactions as leading
component of the total interaction energy between layers has been addressed by
using the Grimme's approach, in which new parameters for Pb and O atoms have
been developed. Both gradient corrected and hybrid functionals have been
adopted using Gaussian-type basis sets of polarized triple zeta quality for O
atoms and small core pseudo-potential for the Pb atoms. Basis set superposition
error (BSSE) has been accounted for by the Boys-Bernardi correction to compute
the interlayer separation. Cross check with calculations adopting plane waves
that are BSSE free have also been performed for both structures and vibrational
frequencies. With the new set of proposed Grimme's type parameters structures
and dynamical parameters for both PbO phases are in good agreement with
experimental data.Comment: 8 pages, 5 figure
Influence of steps on the tilting and adsorption dynamics of ordered Pn films on vicinal Ag(111) surfaces
Here we present a structural study of pentacene (Pn) thin films on vicinal
Ag(111) surfaces by He atom diffraction measurements and density functional
theory (DFT) calculations supplemented with van der Waals (vdW) interactions.
Our He atom diffraction results suggest initial adsorption at the step edges
evidenced by initial slow specular reflection intensity decay rate as a
function of Pn deposition time. In parallel with the experimental findings, our
DFT+vdW calculations predict the step edges as the most stable adsorption site
on the surface. An isolated molecule adsorbs as tilted on the step edge with a
binding energy of 1.4 eV. In addition, a complete monolayer (ML) with
pentacenes flat on the terraces and tilted only at the step edges is found to
be more stable than one with all lying flat or tilted molecules, which in turn
influences multilayers. Hence our results suggest that step edges can trap Pn
molecules and act as nucleation sites for the growth of ordered thin films with
a crystal structure similar to that of bulk Pn.Comment: 4 pages, 4 figures, 1 tabl
Eccrine porocarcinoma of the head: An important differential diagnosis in the elderly patient
Background: Eccrine porocarcinoma is a rare malignant tumor of the sweat gland, characterized by a broad spectrum of clinicopathologic presentations. Surprisingly, unlike its benign counterpart eccrine poroma, eccrine porocarcinoma is seldom found in areas with a high density of eccrine sweat glands, like the palms or soles. Instead, eccrine porocarcinoma frequently occurs on the lower extremities, trunk and abdomen, but also on the head, resembling various other skin tumors, as illustrated in the patients described herein. Observations: We report 5 cases of eccrine porocarcinoma of the head. All patients were initially diagnosed as having epidermal or melanocytic skin tumors. Only after histopathologic examination were they classified as eccrine porocarcinoma, showing features of epithelial tumors with abortive ductal differentiation. Characteristic clinical, histopathologic and immunohistochemical findings of eccrine porocarcinomas are illustrated. Conclusion: Eccrine porocarcinomas are potentially fatal adnexal malignancies, in which extensive metastatic dissemination may occur. Porocarcinomas are commonly overlooked, or misinterpreted as squamous or basal cell carcinomas as well as other common malignant and even benign skin tumors. Knowledge of the clinical pattern and histologic findings, therefore, is crucial for an early therapeutic intervention, which can reduce the risk of tumor recurrence and serious complications. Copyright (c) 2008 S. Karger AG, Basel
First Order Static Excitation Potential: Scheme for Excitation Energies and Transition Moments
We present an approximation scheme for the calculation of the principal
excitation energies and transition moments of finite many-body systems. The
scheme is derived from a first order approximation to the self energy of a
recently proposed extended particle-hole Green's function. A hermitian
eigenvalue problem is encountered of the same size as the well-known Random
Phase Approximation (RPA). We find that it yields a size consistent description
of the excitation properties and removes an inconsistent treatment of the
ground state correlation by the RPA. By presenting a hermitian eigenvalue
problem the new scheme avoids the instabilities of the RPA and should be well
suited for large scale numerical calculations. These and additional properties
of the new approximation scheme are illuminated by a very simple exactly
solvable model.Comment: 15 pages revtex, 1 eps figure included, corrections in Eq. (A1) and
Sec. II
Step-Wise Computational Synthesis of Fullerene C60 derivatives. 1.Fluorinated Fullerenes C60F2k
The reactions of fullerene C60 with atomic fluorine have been studied by
unrestricted broken spin-symmetry Hartree-Fock (UBS HF) approach implemented in
semiempirical codes based on AM1 technique. The calculations were focused on a
sequential addition of fluorine atom to the fullerene cage following indication
of the cage atom highest chemical susceptibility that is calculated at each
step. The effectively-non-paired-electron concept of the fullerene atoms
chemical susceptibility lays the foundation of the suggested computational
synthesis. The obtained results are analyzed from energetic, symmetry, and the
composition abundance viewpoints. A good fitting of the data to experimental
findings proves a creative role of the suggested synthesis methodology.Comment: 33 pages, 11 figures, 2 tables, 2 chart
Risks management and cobots. Identifying critical variables
Trabajo presentado en: 29th European Safety and Reliability Conference (ESREL), 22â26 September 2019, HannoverA collaborative robot or a "Cobot" is the name of a robot that can share a workspace with operators
in the absence of a protective fence or with only partial protection. They represent a new and expanding sector of
industrial robotics. This investigation draws from the latest international rules and safety parameters related to
work with collaborative robots. Its detailed research is motivated by the design of a collaborative industrial robot
system, hazard elimination, risk reduction, and different collaborative operations, such as power and force
limiting, collaborative operation design, and end-effector safety requirements, among others. The purpose of our
study is to analyze the most important variables that must be controlled in accordance with the desired use of the
Cobot, according to ISO / TS 15066, ISO / TR 20218-1and some other generic safety regulations on machines and
industrial robots. A series of observations and appreciations on the use of the Cobot will also be presented
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