1,191 research outputs found
Generalized Wannier functions: a comparison of molecular electric dipole polarizabilities
Localized Wannier functions provide an efficient and intuitive means by which
to compute dielectric properties from first principles. They are most commonly
constructed in a post-processing step, following total-energy minimization.
Nonorthogonal generalized Wannier functions (NGWFs) [Skylaris et al., Phys.
Rev. B 66, 035119 11 (2002); Skylaris et al., J. Chem. Phys. 122, 084119
(2005)] may also be optimized in situ, in the process of solving for the
ground-state density. We explore the relationship between NGWFs and
orthonormal, maximally localized Wannier functions (MLWFs) [Marzari and
Vanderbilt, Phys. Rev. B 56, 12847 (1997); Souza, Marzari, and Vanderbilt,
ibid. 65, 035109 (2001)], demonstrating that NGWFs may be used to compute
electric dipole polarizabilities efficiently, with no necessity for
post-processing optimization, and with an accuracy comparable to MLWFs.Comment: 5 pages, 1 figure. This version matches that accepted for Physical
Review B on 4th May 201
Subspace representations in ab initio methods for strongly correlated systems
We present a generalized definition of subspace occupancy matrices in ab
initio methods for strongly correlated materials, such as DFT+U and DFT+DMFT,
which is appropriate to the case of nonorthogonal projector functions. By
enforcing the tensorial consistency of all matrix operations, we are led to a
subspace projection operator for which the occupancy matrix is tensorial and
accumulates only contributions which are local to the correlated subspace at
hand. For DFT+U in particular, the resulting contributions to the potential and
ionic forces are automatically Hermitian, without resort to symmetrization, and
localized to their corresponding correlated subspace. The tensorial invariance
of the occupancies, energies and ionic forces is preserved. We illustrate the
effect of this formalism in a DFT+U study using self-consistently determined
projectors.Comment: 15 pages, 8 figures. This version (v2) matches that accepted for
Physical Review B on 15th April 201
Ligand Discrimination in Myoglobin from Linear-Scaling DFT+U
Myoglobin modulates the binding of diatomic molecules to its heme group via
hydrogen-bonding and steric interactions with neighboring residues, and is an
important benchmark for computational studies of biomolecules. We have
performed calculations on the heme binding site and a significant proportion of
the protein environment (more than 1000 atoms) using linear-scaling density
functional theory and the DFT+U method to correct for self-interaction errors
associated with localized 3d states. We confirm both the hydrogen-bonding
nature of the discrimination effect (3.6 kcal/mol) and assumptions that the
relative strain energy stored in the protein is low (less than 1 kcal/mol). Our
calculations significantly widen the scope for tackling problems in drug design
and enzymology, especially in cases where electron localization, allostery or
long-ranged polarization influence ligand binding and reaction.Comment: 15 pages, 3 figures. Supplementary material 8 pages, 3 figures. This
version matches that accepted for J. Phys. Chem. Lett. on 10th May 201
Renormalization of myoglobin-ligand binding energetics by quantum many-body effects
We carry out a first-principles atomistic study of the electronic mechanisms
of ligand binding and discrimination in the myoglobin protein. Electronic
correlation effects are taken into account using one of the most advanced
methods currently available, namely a linear-scaling density functional theory
(DFT) approach wherein the treatment of localized iron 3d electrons is further
refined using dynamical mean-field theory (DMFT). This combination of methods
explicitly accounts for dynamical and multi-reference quantum physics, such as
valence and spin fluctuations, of the 3d electrons, whilst treating a
significant proportion of the protein (more than 1000 atoms) with density
functional theory. The computed electronic structure of the myoglobin complexes
and the nature of the Fe-O2 bonding are validated against experimental
spectroscopic observables. We elucidate and solve a long standing problem
related to the quantum-mechanical description of the respiration process,
namely that DFT calculations predict a strong imbalance between O2 and CO
binding, favoring the latter to an unphysically large extent. We show that the
explicit inclusion of many body-effects induced by the Hund's coupling
mechanism results in the correct prediction of similar binding energies for
oxy- and carbonmonoxymyoglobin.Comment: 7 pages, 5 figures. Accepted for publication in the Proceedings of
the National Academy of Sciences of the United States of America (2014). For
the published article see
http://www.pnas.org/content/early/2014/04/09/1322966111.abstrac
Projector self-consistent DFT+U using non-orthogonal generalized Wannier functions
We present a formulation of the density-functional theory + Hubbard model
(DFT+U) method that is self-consistent over the choice of Hubbard projectors
used to define the correlated subspaces. In order to overcome the arbitrariness
in this choice, we propose the use of non-orthogonal generalized Wannier
functions (NGWFs) as projectors for the DFT+U correction. We iteratively refine
these NGWF projectors and, hence, the DFT+U functional, such that the
correlated subspaces are fully self-consistent with the DFT+U ground-state. We
discuss the convergence characteristics of this algorithm and compare
ground-state properties thus computed with those calculated using hydrogenic
projectors. Our approach is implemented within, but not restricted to, a
linear-scaling DFT framework, opening the path to DFT+U calculations on systems
of unprecedented size.Comment: 4 pages, 3 figures. This version (v2) matches that accepted for
Physical Review B Rapid Communications on 26th July 201
How do high performance work systems influence organizational innovation in professional service firms?
Purpose - The aim of this paper is examine how a system of human resource management (HRM) practices, labelled high performance work systems (HPWS), influences organizational innovation in professional service firms (PSFs). In this study, innovation in PSFs is seen as an indicator of firm performance and is calculated as the revenue per person generated from new clients and new services respectively.
Design/methodology/approach– Quantitative data was collected from 195 Managing Partners, HR Managers or experienced Partners in 120 Irish accounting firms. Hierarchical regression analysis was used to test the hypotheses.
Findings - The analysis results indicate strong support for the mediating role of employees’ innovative work behaviours in the relationship between HPWS and two types of PSFs’ innovation performance.
Practical implications - Managers need to effectively adopt and implement innovation-based HRM practices to encourage and support employees’ creative thinking and innovation. Through the adoption and utilization of these practices managers can enhance the firm’s innovation and its performance.
Originality/value - This study contributes to our understanding of the link between HRM and firm innovation by explicating a pathway between these variables. This study also generalizes consistent findings on the HRM-firm innovation relationship to a different context, i.e. professional service firms
Prenatal dexamethasone ‘programmes’ hypotension, but stress-induced hypertension in adult offspring
Low birth weight in humans is predictive of hypertension in adult life. Although the mechanisms underlying this link remain unknown, fetal overexposure to glucocorticoids has been implicated. We previously showed that prenatal dexamethasone (DEX) exposure in the rat lowers birth weight and programmes adult hypertension. The current study aimed to further investigate the nature of this hypertension and to elucidate its origins. Unlike previous studies, we assessed offspring blood pressure (BP) with radiotelemetry, which is unaffected by stress artefacts of measurement. We show that prenatal DEX during the last week of pregnancy results in offspring of low birth weight (14% reduction) that have lower basal BP in adulthood (∼4–8 mmHg lower); with the commonly expected hypertensive phenotype only being noted when these offspring are subjected to even mild disturbance or a more severe stressor (up to 30 mmHg higher than controls). Moreover, DEX-treated offspring sustain their stress-induced hypertension for longer. Promotion of systemic catecholamine release (amphetamine) induced a significantly greater rise of BP in the DEX animals (77% increase) over that observed in the vehicle controls. Additionally, we demonstrate that the isolated mesenteric vasculature of DEX-treated offspring display greater sensitivity to noradrenaline and other vasoconstrictors. We therefore conclude that altered sympathetic responses mediate the stress-induced hypertension associated with prenatal DEX programming
- …