2,877 research outputs found
Non-Empirically Tuned Range-Separated DFT Accurately Predicts Both Fundamental and Excitation Gaps in DNA and RNA Nucleobases
Using a non-empirically tuned range-separated DFT approach, we study both the
quasiparticle properties (HOMO-LUMO fundamental gaps) and excitation energies
of DNA and RNA nucleobases (adenine, thymine, cytosine, guanine, and uracil).
Our calculations demonstrate that a physically-motivated, first-principles
tuned DFT approach accurately reproduces results from both experimental
benchmarks and more computationally intensive techniques such as many-body GW
theory. Furthermore, in the same set of nucleobases, we show that the
non-empirical range-separated procedure also leads to significantly improved
results for excitation energies compared to conventional DFT methods. The
present results emphasize the importance of a non-empirically tuned
range-separation approach for accurately predicting both fundamental and
excitation gaps in DNA and RNA nucleobases.Comment: Accepted by the Journal of Chemical Theory and Computatio
PAMELA: An Open-Source Software Package for Calculating Nonlocal Exact Exchange Effects on Electron Gases in Core-Shell Nanowires
We present a new pseudospectral approach for incorporating many-body,
nonlocal exact exchange interactions to understand the formation of electron
gases in core-shell nanowires. Our approach is efficiently implemented in the
open-source software package PAMELA (Pseudospectral Analysis Method with
Exchange & Local Approximations) that can calculate electronic energies,
densities, wavefunctions, and band-bending diagrams within a self-consistent
Schrodinger-Poisson formalism. The implementation of both local and nonlocal
electronic effects using pseudospectral methods is key to PAMELA's efficiency,
resulting in significantly reduced computational effort compared to
finite-element methods. In contrast to the new nonlocal exchange formalism
implemented in this work, we find that the simple, conventional
Schrodinger-Poisson approaches commonly used in the literature (1) considerably
overestimate the number of occupied electron levels, (2) overdelocalize
electrons in nanowires, and (3) significantly underestimate the relative energy
separation between electronic subbands. In addition, we perform several
calculations in the high-doping regime that show a critical tunneling depth
exists in these nanosystems where tunneling from the core-shell interface to
the nanowire edge becomes the dominant mechanism of electron gas formation.
Finally, in order to present a general-purpose set of tools that both
experimentalists and theorists can easily use to predict electron gas formation
in core-shell nanowires, we document and provide our efficient and
user-friendly PAMELA source code that is freely available at
http://alum.mit.edu/www/usagiComment: Accepted by AIP Advance
Development of a computer model to predict platform station keeping requirements in the Gulf of Mexico using remote sensing data
Offshore operations such as oil drilling and radar monitoring require semisubmersible platforms to remain stationary at specific locations in the Gulf of Mexico. Ocean currents, wind, and waves in the Gulf of Mexico tend to move platforms away from their desired locations. A computer model was created to predict the station keeping requirements of a platform. The computer simulation uses remote sensing data from satellites and buoys as input. A background of the project, alternate approaches to the project, and the details of the simulation are presented
Using access information in the dynamic visualisation of web sites
Includes bibliographical references.Log file analysis provides a cost-effective means to detennine web site usage. However, current methods of displaying log analysis results tend to be limited in that they either contain no reference to a web site's structure, or else they portray this structure as a standard graph or tree. This dissertation presents a visual representation of web server log information, which addresses these limitations by incorporating log file data into a visualisation of a web site's layout. The devised visualisation utilizes properties unique to web sites in order to create a compromise between the clutter-prone network graph and the infonnation incomplete tree representations that have traditionally been used to depict web sites. As such, the visualisation emphasises typical web site features such as the home page, sub-sites and navigation bars. This approach pennitted the introduction of the concept of implying the presence of links without explicitly rendering them. This notion has many implications, not least of which is the reduction of cluttering. The visualisation combined several other techniques to address the issues of structure and data representation, data exploration, scalability and context maintenance. Assessment of the visualisation consisted of a heuristic evaluation by an expert from the web site usage industry, a test to detelmine the intuitiveness of the representation, and a series of user experiments. Results of the assessment were generally promising although a few areas of concern, such as the difficulty experienced by users in navigating the visualisation with a trackball, were identified. These issues should not prove to be too difficult to overcome however. The visualisation could thus be said to have successfully met the aim of developing a representation of web site usage infonnation that incorporates site structure and treats web sites as unique entities, thereby taking advantage of their particular characteristics. It is hoped such a visualisation will be of benefit to web site designers and administrators in analysing and ultimately improving their web sites
The Diamine Cation Is Not a Chemical Example Where Density Functional Theory Fails
In a recent communication, Weber and co-workers presented a surprising study
on charge-localization effects in the N,N'-dimethylpiperazine (DMP+) diamine
cation to provide a stringent test of density functional theory (DFT) methods.
Within their study, the authors examined various DFT methods and concluded that
"all DFT functionals commonly used today, including hybrid functionals with
exact exchange, fail to predict a stable charge-localized state." This
surprising conclusion is based on the authors' use of a self-interaction
correction (namely, complex-valued Perdew-Zunger Self-Interaction Correction
(PZ-SIC)) to DFT, which appears to give excellent agreement with experiment and
other wavefunction-based benchmarks. Since the publication of this recent
communication, the same DMP+ molecule has been cited in numerous subsequent
studies as a prototypical example of the importance of self-interaction
corrections for accurately calculating other chemical systems. In this
correspondence, we have carried out new high-level CCSD(T) analyses on the DMP+
cation to show that DFT actually performs quite well for this system (in
contrast to their conclusion that all DFT functionals fail), whereas the PZ-SIC
approach used by Weber et al. is the outlier that is inconsistent with the
high-level CCSD(T) (coupled-cluster with single and double excitations and
perturbative triples) calculations. Our new findings and analysis for this
system are briefly discussed in this correspondence.Comment: Accepted by Nature Communication
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