Dft Study Of Geometry And Energetics Of Transition Metal Systems

This dissertation focuses on computational study of the geometry and energetics small molecules and nanoclusters involving transition metals (TM). These clusters may be used for various industrial applications including catalysis and photonics. Specifically, in this work we have studied hydrides and carbides of 3d-transition metal systems (Sc through Cu), small nickel and gold clusters. Qualitatively correct description of the bond dissociation is ensured by allowing the spatial and spin symmetry to break. We have tested applicability of new exchange-correlation functional and alternative theoretical descriptions (spin-contamination correction in broken symmetry DFT and ensemble Kohn-Sham (EKS)) as well. We studies TM hydrides and carbides systems to understand the importance of underlying phenomenon of bond breaking in catalytic processes. We have tested several exchange-correlation functionals including explicit dependence on kinetic energy density for the description of hydrides (both neutral and cationic) and carbides formed by 3d-transition metals. We find M05-2x and BMK dissociation energies are in better agreement with experiment (where available) than those obtained with high level wavefunction theory methods, published previously. This agreement with experiment deteriorates quickly for other functionals when the fraction of the Hartree-Fock exchange in DFT functional is decreased. Higher fraction of HF exchange is also essential in EKS formalism, but it does not help when spin-adapted unrestricted approach is employed. We analyze the electron spin densities using Natural Bond Orbital population analysis and find that simple description of 3d electrons as non-bonding in character is rarely correct. Unrestricted formalism results in appreciable spin-contamination for some of the systems at equilibrium, which motivated us to investigate it further in details. In order to correct the spin contamination effect on the energies, we propose a new scheme to correct for spin contamination arising in broken-symmetry DFT approach. Unlike conventional schemes, our spin correction is introduced for each spin-polarized electron pair individually and therefore is expected to yield more accurate energy values. We derive an expression to extract the energy of the pure singlet state from the energy of the broken-symmetry DFT description of the low spin state and the energies of the high spin states (pentuplet and two spin-contaminated triplets in the case of two spin-polarized electron pairs). We validate our spin-contamination correction approach by a simple example of H2 and applied to more complex MnH system. Ensemble KS formalism is also applied to investigate the dissociation of C2 molecule. We find that high fraction of HF exchange is essential to reproduce the results of EKS treatment with exact exchange-correlation functional. We analyze the geometry and energetics of small nickel clusters (Ni2-Ni5) for several lowest energy isomers. We also study all possible spin states of small nickel cluster isomers and report observed trends in energetics. Finally we determine the geometry and energetics of ten lowest energy isomers of four small gold clusters (Au2, Au4, Au6, and Au8). We have also investigated the influence of cluster geometry, ligation, solvation and relativistic effects on electronic structure of these gold clusters. The effect of one-by-one ligand attachment in vacuum and solvent environment is also studied. Performance of five DFT functionals are tested as well; Local Spin Density Approximation (SVWN5), Generalized Gradient Approximation (PBE), kinetic energy density-dependent functional (TPSS), hybrid DFT (B3LYP), and CAM-B3LYP which accounts for long-range exchange effects believed to be important in the analysis of metal bonding in gold complexes and clusters. Our results exhibit the ligand induced stability enhancement of otherwise less stable isomers of Au4, Au6 and Au8. Ligands are found to play a crucial role in determining the 2D to 3D transition realized in small gold clusters. In order to select an appropriate theory level to use in this study, we investigate the effect of attachment of four different ligands (NH3, NMe3, PH3, PMe3) on cluster geometry and energetics of Au2 and Au4 in vacuum and in solution. Our results benchmark the applicability of DFT functional model and polarization functions in the basis set for calculations of ligated gold cluster systems. We employ five different basis sets with increasing amount of polarization and diffuse functions; LANL2DZ, LANL2DZ-P, def2-SVP, def2-TZVP, and def2-QZVP. We obtain NMe3 = NH3 \u3e PH3 \u3e PMe3 order of ligand binding energies and observe shallow potential energy surfaces in all molecules. Our results suggest appropriate quantum-chemical methodologies to model small noble metal clusters in realistic ligand environment to provide reliable theoretical analysis in order to complement experiments

Potential energy curves and electronic structure of 3d transition metal hydrides and their cations

We investigate gas-phase neutral and cationic hydrides formed by 3d transition metals from Sc to Cu with density functional theory (DFT) methods. The performance of two exchange-correlation functionals, Boese-Martin for kinetics (BMK) and Tao-Perdew-Staroverov-Scuseria (TPSS), in predicting bond lengths and energetics, electronic structures, dipole moments, and ionization potentials is evaluated in comparison with available experimental data. To ensure a unique self-consistent field (SCF) solution, we use stability analysis, Fermi smearing, and continuity analysis of the potential energy curves. Broken-symmetry approach was adapted in order to get the qualitatively correct description of the bond dissociation. We found that on average BMK predicted values of dissociation energies and ionization potentials are closer to experiment than those obtained with high level wave function theory methods. This agreement deteriorates quickly when the fraction of the Hartree-Fock exchange in DFT functional is decreased. Natural bond orbital (NBO) population analysis was used to describe the details of chemical bonding in the systems studied. The multireference character in the wave function description of the hydrides is reproduced in broken-symmetry DFT description, as evidenced by NBO analysis. We also propose a new scheme to correct for spin contamination arising in broken-symmetry DFT approach. Unlike conventional schemes, our spin correction is introduced for each spin-polarized electron pair individually and therefore is expected to yield more accurate energy values. We derive an expression to extract the energy of the pure singlet state from the energy of the broken-symmetry DFT description of the low spin state and the energies of the high spin states (pentuplet and two spin-contaminated triplets in the case of two spin-polarized electron pairs). The high spin states are build with canonical natural orbitals and do not require SCF convergence

The Building Blocks of Interoperability. A Multisite Analysis of Patient Demographic Attributes Available for Matching.

BackgroundPatient matching is a key barrier to achieving interoperability. Patient demographic elements must be consistently collected over time and region to be valuable elements for patient matching.ObjectivesWe sought to determine what patient demographic attributes are collected at multiple institutions in the United States and see how their availability changes over time and across clinical sites.MethodsWe compiled a list of 36 demographic elements that stakeholders previously identified as essential patient demographic attributes that should be collected for the purpose of linking patient records. We studied a convenience sample of 9 health care systems from geographically distinct sites around the country. We identified changes in the availability of individual patient demographic attributes over time and across clinical sites.ResultsSeveral attributes were consistently available over the study period (2005-2014) including last name (99.96%), first name (99.95%), date of birth (98.82%), gender/sex (99.73%), postal code (94.71%), and full street address (94.65%). Other attributes changed significantly from 2005-2014: Social security number (SSN) availability declined from 83.3% to 50.44% (p<0.0001). Email address availability increased from 8.94% up to 54% availability (p<0.0001). Work phone number increased from 20.61% to 52.33% (p<0.0001).ConclusionsOverall, first name, last name, date of birth, gender/sex and address were widely collected across institutional sites and over time. Availability of emerging attributes such as email and phone numbers are increasing while SSN use is declining. Understanding the relative availability of patient attributes can inform strategies for optimal matching in healthcare

Modeling Of Selective Carbon Nanotubes Growth For Non-Classical Memory Applications

Single wall carbon nanotubes (SWNT) have unique properties that make them potentially useful in wide variety of applications in nanoelectronics. However, these applications are feasible only if SWNTs have specific chirality. Therefore optimization of experimental conditions for Chemical Vapor Deposition (CVD) growth of SWNT in order to increase its selectivity is of great practical importance. This rational optimization is impossible without knowledge of mechanistic kinetics of CVD. It is not probably feasible to extract the information on mechanism for SWNT synthesis from experimental data. The chemical origin of the reaction barriers and intermediates, however, could be analyzed using molecular simulations. Here we propose multiscale computer modeling of CVD process. Our approach is to extract the structure of the intermediates from molecular dynamics trajectories, conduct the transition state search, predict the free energy activation barriers, build the kinetic model of the growth process, and implement it in kinetic Monte Carlo algorithm to predict the optimal experimental conditions necessary to produce desired chirality of SWNT. © 2009 IEEE

Density Functional Theory Study Of Small Nickel Clusters

Elliptical gain guiding fibers in which gain guiding effects are dominant compared to conventional index guiding fiber is analyzed by solving Mathieu equations with complex-valued fiber parameters. The properties of mode propagation and single mode operation are evaluated in this elliptical gain guiding fiber with the assumption of a uniform gain distribution in the active core medium. Threshold for lossless mode propagation increases exponentially with the eccentricity of the elliptical cross section. Further, the difference in threshold between the lowest two order modes is constant for arbitrary eccentricity. © Springer-Verlag 2011

First Principles Study Of Transition Metal Diatomics As The First Step In Multiscale Simulations Of Carbon Nanotube Growth Process

Single wall carbon nanotubes (SWNT) are cylindrical molecules with unique properties that make them potentially useful in wide variety of applications, including nanoelectronics and photonics. However, these applications are feasible only if SWNTs have specific chirality. Much work remains to be done to gain control over selectivity of SWNT synthesis by chemical vapor deposition (CVD). One of the prime factors affecting the chirality of SWNT is the chemical nature and rate of carbon containing feed gas. Mechanistic kinetics study of CVD processes are gravely complicated by variety of the species involved and by the high temperatures of the reaction chamber. It is not probably feasible to extract the information on mechanism for SWNT synthesis from experimental data. The chemical origin of the reaction barriers and intermediates, however, could be analyzed using molecular simulations. High theory level can be used for di- and tri-atomic fragments, and generate parameters for bond order force field. In turn, force field simulations will be used to characterize intermediates and transition states, and calculate rate constants. Here we propose multiscale computer modeling of CVD process. Our approach is to extract the structure of the intermediates from molecular dynamics trajectories, conduct the transition state search, predict the free energy activation barriers and the rate constants, build the kinetic model of the growth process, and implement it in kinetic Monte Carlo algorithm to predict the optimal experimental conditions necessary to produce desired chirality of SWNT

Pairwise Spin-Contamination Correction Method And Dft Study Of Mnh And H2 Dissociation Curves

A clear advantage of broken symmetry (BS) unrestricted density functional theory DFT is qualitatively correct description of bond dissociation process, but its disadvantage is that spin-polarized Slater determinant is no longer a pure spin state (a.k.a. spin contamination). We propose a new approach to eliminate the spin-contamination, based on canonical Natural Orbitals (NO). We derive an expression to extract the energy of the pure singlet state given in terms of energy of BS DFT solution, the occupation number of the bonding NO, and the energy of the higher state built on these bonding and antibonding NOs (as opposed to self-consistent Kohn-Sham orbitals). Thus, unlike spin-contamination correction schemes by Noodleman and Yamaguchi, spin-correction is introduced for each correlated electron pair individually and thus expected to give more accurate results. We validate this approach on two examples, a simple diatomic H2 and transition metal hydride MnH. © 2009 Springer Berlin Heidelberg

Towards Multiscale Simulations Of Carbon Nanotube Growth Process: A Density Functional Theory Study Of Transition Metal Hydrides

Nanoelectronics and photonics applications of single wall carbon nanotubes (SWNT) are feasible only if SWNTs have specific chirality. The knowledge of the detailed mechanism for SWNT synthesis would allow one to optimize the chemical vapor deposition (CVD) process and may help to gain control over selectivity of SWNT synthesis. While it is not probably feasible to study this mechanism experimentally, it could be analyzed using molecular simulations. Here we propose multiscale computer modeling of CVD process. High theory level can be used for di- and tri-atomic fragments, in order to generate parameters for bond order force field. In turn, force field simulations will be used to characterize the chemical origin and thermochemical properties of the intermediates and transition states. This will allow predicting the rate constants for the elementary steps, which are then used in kinetic Monte Carlo simulations to describe SWNT growth at realistic time scales. © 2009 Springer Berlin Heidelberg

Dissociation Curves And Binding Energies Of Diatomic Transition Metal Carbides From Density Functional Theory

The computational description of the catalytic processes on the surface of transition metals (TMs) requires methods capable of accurate prediction of the bond forming and breaking between the atoms of metal and other elements. In our previous report [Goel and Masunov, J Chem Phys, 129, 214302, 2008], we studied TM hydrides and found that Boese-Martin functional for kinetics (BMK) combined with broken symmetry approach described dissociation process more accurately than multireference wavefunction theory (WFT) methods and some other functionals. Here, we investigate the binding energy, geometry, electronic structure, and potential energy curves for diatomic TM carbides using several exchange-correlation functionals. The functionals that include explicit dependence on the kinetic energy density (τ-functionals) are considered, among others. We have found M05-2x performance to be the best, followed by BMK, when compared with experimental and high level WFT energetics. This agreement deteriorates quickly for other functionals when the fraction of the Hartree-Fock exchange is decreased. Scalar relativistic corrections yield mixed results for bond lengths and bond energies. The natural bond orbital analysis provides useful insight in description of stable spin state over others in these diatomics. © 2011 Wiley Periodicals, Inc