arXiv subjects

Rajendra R. Zope

Publications and source records attributed to Rajendra R. Zope.

49 records · Page 3Linked to original sources

arXiv2005

Momentum space properties from coordinate space electron density

Electron density and electron momentum density, while independently tractable experimentally, bear no direct connection without going through the many-electron wave function. However, invoking a variant of the constrained-search formulation of density functional theory, we develop a general scheme (valid for arbitrary external potentials) yielding decent momentum space properties, starting exclusively from the coordinate space electron density. Numerical illustration of the scheme is provided for the closed-shell atomic systems He, Be and Ne and for $1s^1~2s^1$ singlet electronic excited state for Helium by calculating the Compton profiles and the $ $ expectation values derived from given coordinate space electron densities.

physics.atom-ph↗

arXiv2005

Theoretical infra-red, Raman, and Optical spectra of the B36N36 cage

The B36N36 fullerene-like cage structure was proposed as candidate structure for the single-shell boron-nitride cages observed in electron-beam irradiation experiment. We have performed all electron density functional calculations, with large polarized Gaussian basis sets, on the B36N36 cage. We show that the cage is energetically and vibrationally stable. The infra-red, Raman and optical spectra are calculated. The predicted spectra, in combination with experimentally measured spectra, will be useful in conclusive assignment of the proposed B36N36 cage. The vertical and adiabatic ionization potentials as well as static dipole polarizability are also reported.

physics.atm-clus↗

arXiv2004

On optimal values of alpha for the analytic Hartree-Fock-Slater method

We have examined the performance of the analytic Hartree-Fock-Slater (HFS) method for various alpha (Slater's exchange parameter) values and empiricaly determined the optimal alpha value by minimizing the mean absolute error (MAE) in atomization energies of the G2 set of molecules. At the optimal alpha the HFS method's performance is far superior with the MAE of 14 kcal/mol than that of the local density approximation (MAE ~ 36 kcal/mol) or the Hartree-Fock theory (MAE ~ 78 kcal/mol). The HFS exchange functional with alpha = 0.7091 is found to perform significantly better than the Kohn-Sham exchange functional for equally weighted atoms H-Kr. We speculate that use of this alpha value may be useful in parametrization of empirical exchange-correlation functionals.

cond-mat.other↗

arXiv2004

Electronic structure, vibrational stability, infra-red, and Raman spectra of B24N24 cages

We examine the vibrational stability of three candidate structures for the B24N24 cage and report their infra-red (IR) and Raman spectra. The candidate structures considered are a round cage with octahedral O symmetry, a cage with S_4 symmetry that satisfies the isolated square rule, and a cage of S_8 symmetry, which combines the caps of the (4,4) nanotube, and contains two extra squares and octagons. The calculations are performed within density functional theory, at the all electron level, with large basis sets, and within the generalized gradient approximation. The vertical ionization potential (VIP) and static dipole polarizability are also reported. The S_4 and S_8 cages are energetically nearly degenerate and are favored over the O cage which has six extra octagons and squares. The IR and Raman spectra of the three clusters show notable differences providing thereby a way to identify and possibly synthesize the cages.

physics.atm-clus↗

arXiv2004

Are hemispherical caps of boron-nitride nanotubes possible?

We report all-electron, density-functional calculations with large Gaussian polarization basis set of the recently synthesized octahedral B24N24 cage that is perfectly round by symmetry, and boron-nitride (BN) clusters that its existence might suggest. We consider whether it is energetically possible that the two halves of this round cage could cap the BN nanotubes, modeled by B28N28 and B32N32. The energetics show that BN nanotubes with such round caps, are only slightly less favorable than the BN clusters containing six squares as the only defects in the otherwise perfect hexagonal lattice. A larger B96N96 octahedral cage formed from B24N24 by adding sufficient hexagons to isolate all squares is not very favorable energetically. The squares protrude noticeably from its otherwise round surface.

physics.atm-clus↗

arXiv2004

Stability of As$_n$ [n=4, 8, 20, 28, 32, 36, 60] Cage Structures

We present all-electron density functional study of the geometry, electronic structure, vibrational modes, polarizabilities as well as the infrared and Raman spectra of fullerene-like arsenic cages. The stability of As$_n$ cages for sizes 4, 8, 20, 28, 32, 36, and 60 wherein each As atom is three-fold coordinated is examined. We find that all the cages studied are vibrationally stable and while all the clusters are energetically stable with respect to isolated arsenic atoms, only As$_{20}$ is energetically stable against dissociation into As$_4$. We suggest that the Raman spectra might be a means for observing the As$_{20}$ molecule in gas phase.

physics.atm-clus↗

arXiv2004

Molecular structures and vibrations of neutral and anionic CuOx (x = 1-3,6) clusters

We report equilibrium geometric structures of CuO2, CuO3, CuO6, and CuO clusters obtained by an all-electron linear combination of atomic orbitals scheme within the density-functional theory with generalized gradient approximation to describe the exchange-correlation effects. The vibrational stability of all clusters is examined on the basis of the vibrational frequencies. A structure with Cs symmetry is found to be the lowest-energy structure for CuO2, while a -shaped structure with C2v symmetry is the most stable structure for CuO3. For the larger CuO6 and CuO clusters, several competitive structures exist with structures containing ozonide units being higher in energy than those with O2 units. The infrared and Raman spectra are calculated for the stable optimal geometries. ~

physics.atm-clus↗

arXiv2003

Electronic structure and rebonding in the onion-like As@Ni12@As20 cluster

We present the first ab initio study of the geometry, electronic structure, charged states, bonding and vibrational modes of the recently synthesized fullerene-like As@Ni12@As20 cluster which has icosahedral point symmetry [Science, 300, 778 (2003)]. We show that the molecule is vibrationally stable and will be electronically most stable in its -3 oxidation state in the condensed phase and in -2 state in the gas phase. We examine the bonding in this unusually structured molecule from charge transfer between atoms, infrared and Raman spectra, and charge density isosurfaces.

cond-mat.mtrl-sci↗

arXiv2003

Interatomic potentials for atomistic simulations of the Ti-Al system

Semi-empirical interatomic potentials have been developed for Al, alpha-Ti, and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large database of experimental as well as ab-initio data. The ab-initio calculations were performed by the linear augmented plane wave (LAPW) method within the density functional theory to obtain the equations of state for a number of crystal structures of the Ti-Al system. Some of the calculated LAPW energies were used for fitting the potentials while others for examining their quality. The potentials correctly predict the equilibrium crystal structures of the phases and accurately reproduce their basic lattice properties. The potentials are applied to calculate the energies of point defects, surfaces, planar faults in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al system, the proposed potentials provide reasonable description of the lattice thermal expansion, demonstrating their usefulness in the molecular dynamics or Monte Carlo studies at high temperatures. The energy along the tetragonal deformation path (Bain transformation) in gamma-TiAl calculated with the EAM potential is in a fairly good agreement with LAPW calculations. Equilibrium point defect concentrations in gamma-TiAl are studied using the EAM potential. It is found that antisite defects strongly dominate over vacancies at all compositions around stoichiometry, indicating that gamm-TiAl is an antisite disorder compound in agreement with experimental data.

cond-mat.mtrl-sci↗

arXiv2001

Charge-Induced Fragmentation of Sodium Clusters

The fission of highly charged sodium clusters with fissilities X>1 is studied by {\em ab initio} molecular dynamics. Na_{24}^{4+} is found to undergo predominantly sequential Na_{3}^{+} emission on a time scale of 1 ps, while Na_{24}^{Q+} (5 \leq Q \leq 8) undergoes multifragmentation on a time scale \geq 0.1 ps, with Na^{+} increasingly the dominant fragment as Q increases. All singly-charged fragments Na_{n}^{+} up to size n=6 are observed. The observed fragment spectrum is, within statistical error, independent of the temperature T of the parent cluster for T \leq 1500 K. These findings are consistent with and explain recent trends observed experimentally.

physics.atm-clus↗

arXiv1999

Full potential LAPW calculation of electron momentum density and related properties of Li

Electron momentum density and Compton profiles in Lithium along $<100 >$, $<110>$, and $<111>$ directions are calculated using Full-Potential Linear Augmented Plane Wave basis within generalized gradient approximation. The profiles have been corrected for correlations with Lam-Platzman formulation using self-consistent charge density. The first and second derivatives of Compton profiles are studied to investigate the Fermi surface breaks. Decent agreement is observed between recent experimental and our calculated values. Our values for the derivatives are found to be in better agreement with experiments than earlier theoretical results. Two-photon momentum density and one- and two-dimensional angular correlation of positron annihilation radiation are also calculated within the same formalism and including the electron-positron enhancement factor.

cond-mat.mtrl-sci↗

arXiv1999

Atomic Compton profiles within different exchange-only theories

The Impulse Compton Profiles (CP's) J(q) and the - expectation values for some inert gas atoms (He-Kr) are computed and compared within the Harbola-Sahni (HS), Hartree-Fock(HF) theories and a Self Interaction Corrected (SIC) density functional model. The Compton profiles for excited states of Helium atom are also calculated. While the calculated CP's are found to generally agree, they differ slightly from one another for small values of the Compton parameter q and are in good agreement for large q values. The expectation values within the three theories are also found to be comparable. The HS formalism seem to mimic HF reasonably well in the momentum space, establishing the logical consistency of the former.

physics.atom-ph↗

arXiv1999

Positron and positronium affinities in the work-formalism Hartree-Fock approximation

Positron binding to anions is investigated within the work formalism proposed by Harbola and Sahni for the halide anions and the systems Li^- through O^- excluding Be^- and N^-. The toal ground-state energies of the anion-positron bound systems are empirically found to be an upper bound to the Hartree-Fock energies. The computed expectation values as well as positron and positronium affinities are in good agreement with their restricted Hartree-Fock counterparts. Binding of a positron to neutral species is also investigated using an iterative method.

physics.atom-ph↗