Biomedical subjects

Gustavo E Scuseria

Publications and source records attributed to Gustavo E Scuseria.

At least 19 recordsLinked to original sources

PubMed2006

Assessment of a long-range corrected hybrid functional.

Common approximate exchange-correlation functionals suffer from self-interaction error, and as a result, their corresponding potentials have incorrect asymptotic behavior. The exact asymptote can be imposed by introducing range separation into the exchange component and replacing the long-range portion of the approximate exchange by the Hartree-Fock counterpart. The authors show that this long-range correction works particularly well in combination with the short-range variant of the Perdew-Burke-Ernzerhof (PBE) exchange functional. This long-range-corrected hybrid, here denoted LC-omegaPBE, is remarkably accurate for a broad range of molecular properties, such as thermochemistry, barrier heights of chemical reactions, bond lengths, and most notably, description of processes involving long-range charge transfer.

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PubMed2006

Influence of the exchange screening parameter on the performance of screened hybrid functionals.

This work reexamines the effect of the exchange screening parameter omega on the performance of the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. We show that variation of the screening parameter influences solid band gaps the most. Other properties such as molecular thermochemistry or lattice constants of solids change little with omega. We recommend a new version of HSE with the screening parameter omega=0.11 bohr(-1) for further use. Compared to the original implementation, the new parametrization yields better thermochemical results and preserves the good accuracy for band gaps and lattice constants in solids.

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PubMed2006

Linear-scaling calculation of static and dynamic polarizabilities in Hartree-Fock and density functional theory for periodic systems.

We present a linear-scaling method for analytically calculating static and dynamic polarizabilities with Hartree-Fock and density functional theory, using Gaussian orbitals and periodic boundary conditions. Our approach uses the direct space fast multipole method to evaluate the long-range Coulomb contributions. For exact exchange, we use efficient screening techniques developed for energy calculations. We then demonstrate the capabilities of our approach with benchmark calculations on one-, two-, and three-dimensional systems.

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PubMed2006

Spurious fractional charge on dissociated atoms: pervasive and resilient self-interaction error of common density functionals.

Semilocal density functional approximations for the exchange-correlation energy can improperly dissociate a neutral molecule XY (Y not =X) to fractionally charged fragments X(+q)...Y(-q) with an energy significantly lower than X0...Y0. For example, NaCl can dissociate to Na(+0.4)...Cl(-0.4). Generally, q is positive when the lowest-unoccupied orbital energy of atom Y0 lies below the highest-occupied orbital energy of atom X0. The first 24 open sp-shell atoms of the Periodic Table can form 276 distinct unlike pairs XY, and in the local spin density approximation 174 of these display fractional-charge dissociation. Finding these lowest-energy solutions with standard quantum chemistry codes, however, requires special care. Self-interaction-corrected (SIC) semilocal approximations are exact for one-electron systems and also reduce the spurious fractional charge q. The original SIC of Perdew and Zunger typically reduces q to 0. A scaled-down SIC with better equilibrium properties sometimes fails to reduce q all the way to 0. The desideratum of "many-electron self-interaction freedom" is introduced as a generalization of the one-electron concept.

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PubMed2006

Surface enhanced Raman optical activity of molecules on orientationally averaged substrates: theory of electromagnetic effects.

We present a model for electromagnetic enhancements in surface enhanced Raman optical activity (SEROA) spectroscopy. The model extends previous treatments of SEROA to substrates, such as metal nanoparticles in solution, that are orientationally averaged with respect to the laboratory frame. Our theoretical treatment combines analytical expressions for unenhanced Raman optical activity with molecular polarizability tensors that are dressed by the substrate's electromagnetic enhancements. We evaluate enhancements from model substrates to determine preliminary scaling laws and selection rules for SEROA. We find that dipolar substrates enhance Raman optical activity (ROA) scattering less than Raman scattering. Evanescent gradient contributions to orientationally averaged ROA scale to first or higher orders in the gradient of the incident plane-wave field. These evanescent gradient contributions may be large for substrates with quadrupolar responses to the plane-wave field gradient. Some substrates may also show a ROA contribution that depends only on the molecular electric dipole-electric dipole polarizability. These conclusions are illustrated via numerical calculations of surface enhanced Raman and ROA spectra from (R)-(-)-bromochlorofluoromethane on various model substrates.

Electromagnetic Phenomena↗

PubMed2006

Theoretical nitrogen NMR chemical shifts in octahedral boron nitride cages.

We have calculated the geometrical structure, relative stability, and nitrogen chemical shifts of five boron nitride hollow octahedral cages using density functional theory. Our results show three typical ranges for nitrogen chemical shifts corresponding to each of the nonequivalent magnetic sites of the N atoms. The principal component of the electric field gradient tensor at each 14N site in boron nitride cages is predicted to be much smaller than the corresponding value in borazine, which should reflect in sharper spectral lines and much better resolution.

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PubMed2006

First-principles electronic transport calculations in finite elongated systems: a divide and conquer approach.

We present a first-principles method for the evaluation of the transmittance probability and the coherent conductance through elongated systems composed of a repeating molecular unit and terminated at both ends. Our method is based on a divide and conquer approach in which the Hamiltonian of the elongated system can be represented by a block tridiagonal matrix, and therefore can be readily inverted. This allows us to evaluate the transmittance and the conductance using first-principles electronic structure methods without explicitly performing calculations involving the entire system. A proof of concept model based on a trans-polyacetylene chain bridging two aluminum leads indicates that our divide and conquer approach is able to capture all the features appearing in the transmittance probability curves obtained by a full scale calculation.

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PubMed2006

Efficient evaluation of short-range Hartree-Fock exchange in large molecules and periodic systems.

We present an efficient algorithm for the evaluation of short-range Hartree-Fock exchange energies and geometry gradients in Gaussian basis sets. Our method uses a hierarchy of screening levels to eliminate negligible two-electron integrals whose evaluation is the fundamental computational bottleneck of the procedure. By applying our screening technique to the Heyd-Scuseria-Ernzerhof [J. Chem. Phys. 118, 8207 (2003)] short-range Coulomb hybrid density functional, we achieve a computational efficiency comparable with that of standard nonhybrid density functional calculations.

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PubMed2006

Effective local potentials for orbital-dependent density functionals.

Practicality of the Kohn-Sham density functional scheme for orbital-dependent functionals hinges on the availability of an efficient procedure for constructing local exchange-correlation potentials in finite basis sets. We have shown recently that the optimized effective potential (OEP) method, commonly used for this purpose, is not free from difficulties. Here we propose a robust alternative to OEPs, termed effective local potentials (ELPs), based on minimizing the variance of the difference between a given nonlocal potential and its desired local counterpart. The ELP method is applied to the exact-exchange-only problem and shown to be promising for overcoming troubles with OEPs.

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PubMed2006

Importance of short-range versus long-range Hartree-Fock exchange for the performance of hybrid density functionals.

We consider a general class of hybrid density functionals with decomposition of the exchange component into short-range and long-range parts. The admixture of Hartree-Fock (HF) exchange is controlled by three parameters: short-range mixing, long-range mixing, and range separation. We study how the variation of these parameters affects the accuracy of hybrid functionals for thermochemistry and kinetics. For the density functional component of the hybrids, we test three nonempirical approximations: local spin-density approximation, generalized gradient approximation (GGA), and meta-GGA. We find a great degree of flexibility in choosing the mixing parameters in range-separated hybrids. For the studied properties, short-range and long-range HF exchange seem to have a similar effect on the errors. One may choose to treat the long-range portion of the exchange by HF to recover the correct asymptotic behavior of the exchange potential and improve the description of density tail regions. If this asymptote is not important, as in solids, one may use screened hybrids, where long-range HF exchange is excluded. Screened hybrids retain most of the benefits of global hybrids but significantly reduce the computational cost in extended systems.

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PubMed2006

Theoretical study of CeO2 and Ce2O3 using a screened hybrid density functional.

The predicted structures and electronic properties of CeO(2) and Ce(2)O(3) have been studied using conventional and hybrid density functional theory. The lattice constant and bulk modulus for CeO(2) from local (LSDA) functionals are in good agreement with experiment, while the lattice parameter from a generalized gradient approximation (GGA) is too long. This situation is reversed for Ce(2)O(3), where the LSDA lattice constant is much too short, while the GGA result is in reasonable agreement with experiment. Significantly, the screened hybrid HSE functional gives excellent agreement with experimental lattice constants for both CeO(2) and Ce(2)O(3). All methods give insulating ground states for CeO(2) with gaps for the 4f band lying between 1.7 eV (LSDA) and 3.3 eV (HSE) and 6-8 eV for the conduction band. For Ce(2)O(3) the local and GGA functionals predict a semimetallic ground state with small (0-0.3 eV) band gap but weak ferromagnetic coupling between the Ce(+3) centers. By contrast, the HSE functional gives an insulating ground state with a band gap of 3.2 eV and antiferromagnetic coupling. Overall, the hybrid HSE functional gives a consistent picture of both the structural and electronic properties of CeO(2) and Ce(2)O(3) while treating the 4f band consistently in both oxides.

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PubMed2006

Longitudinal polarizability of carbon nanotubes.

The longitudinal polarizabilities of carbon nanotubes are determined using first principles density functional theory. These results demonstrate that the polarizability per atom of a nanotube in the axial direction is primarily determined by the band gap. In fact, polarizability per atom versus inverse band gap yields a linear trend for all nanotubes and methods utilized in this study, creating a universal relationship for longitudinal polarizability. This can be explained by examining the terms in the sum over states equation used to determine polarizability and noting that the vast majority of the polarizability arises from a few elements near the band gap. This universal trend is then used with experimentally determined band gaps to predict the experimental polarizability of carbon nanotubes.

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PubMed2006

A simple method to selectively scale down the self-interaction correction.

The method of Perdew and Zunger is commonly used to correct the self-interaction error of approximate density functionals. However, it has been shown that this orbital-based self-interaction correction (SIC) tends to overcorrect and often impairs molecular properties. We have recently proposed a method to improve the performance of the SIC by scaling it down in many-electron regions. In this communication, we present a simplification of this scheme. For every occupied orbital, we introduce a scaling factor determined by the ratio of the orbital density to the total spin-density. Thus, the magnitude of the correction is adjusted depending on how much orbital densities overlap with one another. Such a modification of the Perdew-Zunger SIC does not add any appreciable time to the computation, but significantly improves the accuracy for a number of benchmark properties.

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PubMed2006

Optimized effective potentials yielding Hartree-Fock energies and densities.

It is commonly believed that the exchange-only optimized effective potential (OEP) method must yield total energies that are above corresponding ground-state Hartree-Fock (HF) energies except for one- and two-electron systems. We present a simple procedure for constructing local (multiplicative) exchange potentials that reproduce exactly the HF energy and density in any finite basis set for any number of electrons. For any finite basis set, no matter how large, there exist infinitely many such OEPs, which questions their suitability for practical applications.

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PubMed2006

Scaling down the Perdew-Zunger self-interaction correction in many-electron regions.

Semilocal density functional approximations (DFAs) for the exchange-correlation energy suffer from self-interaction error, which is believed to be the cause of many of the failures of common DFAs, such as poor description of charge transfer and transition states of chemical reactions. The standard self-interaction correction (SIC) of Perdew and Zunger mends some of these failures but spoils such essential properties as thermochemistry and equilibrium bond lengths. The Perdew-Zunger SIC seems to overcorrect many-electron systems. In this paper, we propose a modified SIC, which is scaled down in many-electron regions. The new SIC has an improved performance for many molecular properties, including total energies, atomization energies, barrier heights of chemical reactions, ionization potentials, electron affinities, and bond lengths. The local spin-density approximation (LSDA) benefits from SIC more than higher-level functionals do. The scaled-down SIC has only one adjustable parameter. Rationalization of the optimal value of this parameter enables us to construct an almost-nonempirical version of the scaled-down SIC-LSDA, which is significantly better than uncorrected LSDA and even better than the uncorrected generalized gradient approximation. We present an analysis of the formal properties of the scaled-down SIC and define possible directions for further improvements. In particular, we find that exactness for all one-electron densities does not guarantee correct asymptotics for the exchange-correlation potential of a many-electron system.

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PubMed2006

Coaxial carbon nanotubes as shielded nanowires.

We report transverse polarizabilities of coaxial carbon nanotubes using first principles density functional theory. These results demonstrate a shielding of the inner nanotube from electric fields by the outer nanotube. This study has implications for nanoelectronics, specifically for the possibility of using coaxial nanotubes as nanoelectrical wires. Shielding is predicted to be on the order of 95% by high-level polarizability calculations. This shielding occurs regardless of whether the outer nanotube is metallic or semiconducting. In addition, a series of calculations on coaxial nanotubes where the inner nanotube is not centered show that the shielding still occurs with approximately the same magnitude. These calculations therefore indicate that it would be possible to use a coaxial carbon nanotube as a shielded nanowire.

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PubMed2006

Screened exchange hybrid density-functional study of the work function of pristine and doped single-walled carbon nanotubes.

We present a detailed study of the work function of pristine and doped single-walled carbon nanotubes (SWCNTs) using a novel screened exchange hybrid density functional. We find that SWCNTs with diameters larger than 0.9 nm tend asymptotically and smoothly to the graphene limit of 4.6 eV. On the other hand, the work function of narrow tubes exhibits a strong dependence on their diameter and chiral angle. Boron or nitrogen doping, with concentrations from 1% to 2%, not only changes the electronic behavior by introducing new states around the Fermi level, but also produces a significant change of the work function that can vary between 3.9 (N doping) and 5.2 eV (B doping).

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PubMed2006

Electronic structure and stability of semiconducting graphene nanoribbons.

We present a systematic density functional theory study of the electronic properties, optical spectra, and relative thermodynamic stability of semiconducting graphene nanoribbons. We consider ribbons with different edge nature including bare and hydrogen-terminated ribbons, several crystallographic orientations, and widths up to 3 nm. Our results can be extrapolated to wider ribbons providing a qualitative way of determining the electronic properties of ribbons with widths of practical significance. We predict that in order to produce materials with band gaps similar to Ge or InN, the width of the ribbons must be between 2 and 3 nm. If larger bang gap ribbons are needed (like Si, InP, or GaAs), their width must be reduced to 1-2 nm. According to the extrapolated inverse power law obtained in this work, armchair carbon nanoribbons of widths larger than 8 nm will present a maximum band gap of 0.3 eV, while for ribbons with a width of 80 nm the maximum possible band gap is 0.05 eV. For chiral nanoribbons the band gap oscillations rapidly vanish as a function of the chiral angle indicating that a careful design of their crystallographic nature is an essential ingredient for controlling their electronic properties. Optical excitations show important differences between ribbons with and without hydrogen termination and are found to be sensitive to the carbon nanoribbon width. This should provide a practical way of revealing information on their size and the nature of their edges.

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