PubMed Health⌕ Search

Biomedical subjects

Ali Alavi

Publications and source records attributed to Ali Alavi.

9 recordsLinked to original sources
PubMed2006

A first-principles investigation of LiNH(2) as a hydrogen-storage material: effects of substitutions of K and Mg for Li.

Li-N-H compounds hold promise as novel hydrogen-storage materials with high gravimetric hydrogen densities. Because the dehydriding reaction caused by the decomposition of LiNH(2) requires a higher temperature than desired, much effort has been devoted to the destabilization of LiNH(2) to decrease the decomposition temperature. In particular, there has been recent experimental evidence for lowering the temperature by partial substitution of Li by Mg. However, the reason is not clear. In this study, we have employed density functional theory to investigate LiNH(2) and partially Li-substituted systems aiming to understand the effects of the substitution on the destabilization of the NH(2) species. K, a more electropositive element, and Mg, a more electronegative element, have been chosen as two probes to illustrate the effects. We have focused on the investigation of effects of substitutions on the N-H bond strength that is regarded as a qualitative indicator of the decomposition temperature. We have found that in both cases the N-H bonds are weakened, in particular, the Mg substitute appears to be more effective in the destabilization of the NH(2). The relative strength of the metal-N ionic bonding has been found to be a key factor to explain the effects of the substitutes. These have been discussed in detail in terms of Wannier function analyses.

Journal Article↗
PubMed2005

Quantum delocalization of hydrogen in the Li2NH crystal.

By mapping out potential energy surfaces from density-functional theory (DFT) and solving a protonic Schrödinger equation, we find that the H atom in a unit cell of the Li2NH crystal shows remarkably strong quantum behavior, leading to the delocalization of H over six octahedral sites around each N. This can be rationalized in terms of rapid coherent tunneling among these equivalent octahedral sites. Structural and dynamical consequences of our finding are discussed. Since the Li-N-H compounds are considered promising candidates for H-storage, understanding of these fundamental properties will be useful toward improving the performance of the material.

Journal Article↗
PubMed2005

A combinatorial approach to the electron correlation problem.

Starting from a path-integral formulation of quantum statistical mechanics expressed in a space of Slater determinants, we develop a method for the Monte Carlo evaluation of the energy of a correlated electronic system. The path-integral expression for the partition function is written as a contracted sum over graphs. A graph is a set of distinct connected determinants on which paths can be represented. The weight of a graph is given by the sum over exponentially large numbers of paths which visit the vertices of the graph. We show that these weights are analytically computable using combinatorial techniques, and they turn out to be sufficiently well behaved to allow stable Monte Carlo simulations in which graphs are stochastically sampled according to a Metropolis algorithm. In the present formulation, graphs of up to four vertices have been included. In a Hartree-Fock basis, this allows for paths which include up to sixfold excitations relative to the Hartree-Fock determinant. As an illustration, we have studied the dissociation curve of the N(2) molecule in a VDZ basis, which allows comparison with full configuration-interaction calculations.

Journal Article↗
PubMed2005

First-principles study of superabundant vacancy formation in metal hydrides.

Recent experiments have established the generality of superabundant vacancies (SAV) formation in metal hydrides. Aiming to elucidate this intriguing phenomenon and to clarify previous interpretations, we employ density-functional theory to investigate atomic mechanisms of SAV formation in fcc hydrides of Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au. We have found that upon H insertion, vacancy formation energies reduce substantially. This is consistent with experimental suggestions. We demonstrate that the entropy effect, which has been proposed to explain SAV formation, is not the main cause. Instead, it is the drastic change of electronic structure induced by the H in the SAV hydrides, which is to a large extent responsible. Interesting trends in systems investigated are also found: ideal hydrides of 5d metals and noble metals are unstable compared to the corresponding pure metals, but the SAV hydrides are more stable than the corresponding ideal hydrides, whereas opposite results exist in the cases of Ni, Rh, and Pd. These trends of stabilities of the SAV hydrides are discussed in detail and a general understanding for SAV formation is provided. Finally, we propose an alternative reaction pathway to generate a SAV hydride from a metal alloy.

Journal Article↗
PubMed2005

A comparison of Hartree-Fock and exact diagonalization solutions for a model two-electron system.

We compare the restricted (RHF) and unrestricted (UHF) Hartree-Fock solutions with the exact configuration interaction (CI) solutions for the model problem of two interacting electrons confined within an infinite spherical potential of radius R. An investigation of the ground state energy, electron density, and a population analysis of the wave functions reveals that the UHF solution compares favorably with the CI one in both the small and large R limits. The Coulson-Fischer transition (the point at which the RHF and UHF solutions are no longer degenerate) occurs at an r(s) identical with R/2(1/3) approximately 6 a.u., close to the crossover r(s) where the exact CI density acquires a minimum at the sphere center; this behavior captures the onset of Wigner molecule formation. In this regime, however, the HF densities and energies are not quantitatively accurate when compared to the CI ones. This failure is further quantified by computing Ec/E to determine the ability of each method at capturing the exact correlation energy; we conclude that the UHF solutions can be reasonably accurate at both high and low densities, but not at intermediate r(s).

Journal Article↗
PubMed2003

New insights into ethene epoxidation on two oxidized Ag[111] surfaces.

Reaction mechanisms and activation energies for the complete conversion of ethene to ethene epoxide on two recently characterized oxidized Ag{111} surfaces have been determined from density functional theory. On both surfaces, epoxidation proceeds through a two-step nonconcerted mechanism via an oxametallacycle intermediate. The key implications are that both surfaces are active and that epoxidation can take place over a wide O coverage regime.

Journal Article↗
PubMed2003

Identification of general linear relationships between activation energies and enthalpy changes for dissociation reactions at surfaces.

The activation energy to reaction is a key quantity that controls catalytic activity. Having used ab inito calculations to determine an extensive and broad ranging set of activation energies and enthalpy changes for surface-catalyzed reactions, we show that linear relationships exist between dissociation activation energies and enthalpy changes. Known in the literature as empirical Brønsted-Evans-Polanyi (BEP) relationships, we identify and discuss the physical origin of their presence in heterogeneous catalysis. The key implication is that merely from knowledge of adsorption energies the barriers to catalytic elementary reaction steps can be estimated.

Journal Article↗
PubMed2002

Catalytic role of gold in gold-based catalysts: a density functional theory study on the CO oxidation on gold.

Gold-based catalysts have been of intense interests in recent years, being regarded as a new generation of catalysts due to their unusually high catalytic performance. For example, CO oxidation on Au/TiO(2) has been found to occur at a temperature as low as 200 K. Despite extensive studies in the field, the microscopic mechanism of CO oxidation on Au-based catalysts remains controversial. Aiming to provide insight into the catalytic roles of Au, we have performed extensive density functional theory calculations for the elementary steps in CO oxidation on Au surfaces. O atom adsorption, CO adsorption, O(2) dissociation, and CO oxidation on a series of Au surfaces, including flat surfaces, defects and small clusters, have been investigated in detail. Many transition states involved are located, and the lowest energy pathways are determined. We find the following: (i) the most stable site for O atom on Au is the bridge site of step edge, not a kink site; (ii) O(2) dissociation on Au (O(2)-->2O(ad)) is hindered by high barriers with the lowest barrier being 0.93 eV on a step edge; (iii) CO can react with atomic O with a substantially lower barrier, 0.25 eV, on Au steps where CO can adsorb; (iv) CO can react with molecular O(2) on Au steps with a low barrier of 0.46 eV, which features an unsymmetrical four-center intermediate state (O-O-CO); and (v) O(2) can adsorb on the interface of Au/TiO(2) with a reasonable chemisorption energy. On the basis of our calculations, we suggest that (i) O(2) dissociation on Au surfaces including particles cannot occur at low temperatures; (ii) CO oxidation on Au/inactive-materials occurs on Au steps via a two-step mechanism: CO+O(2)-->CO(2)+O, and CO+O-->CO(2); and (iii) CO oxidation on Au/active-materials also follows the two-step mechanism with reactions occurring at the interface.

Journal Article↗