arXiv subjects

Mark R. Pederson

Publications and source records attributed to Mark R. Pederson.

43 records · Page 3Linked to original sources

arXiv2002

Predicted Infrared and Raman Spectra for Neutral Ti_8C_12 Isomers

Using a density-functional based algorithm, the full IR and Raman spectra are calculated for the neutral Ti_8C_12 cluster assuming geometries of Th, Td, D2d and C3v symmetry. The Th pentagonal dodecahedron is found to be dynamically unstable. The calculated properties of the relaxed structure having C3v symmetry are found to be in excellent agreement with experimental gas phase infrared results, ionization potential and electron affinity measurements. Consequently, the results presented may be used as a reference for further experimental characterization using vibrational spectroscopy.

cond-mat.mtrl-sci↗

arXiv2002

Stability, Electronic Structure and Vibrational Modes of Ti_8C_12 Dimer

We present our density functional results of the geometry, electronic structure and dissociation energy of Ti_8C_12 dimer. We show that as opposed to the currently held view that Ti_8C_12 are highly stable monodispersed clusters, the neutral Ti_8C_12 clusters form covalent bonds and form stable dimers. We determine that the Ti atoms bond weakly (0.9 eV/bond) to organic ligands such as ammonia. Alternatively the Met-Car dimer has a cohesive energy of 4.84 eV or approximately 1.2 eV per bond. While Met-Car dimers are stable, formation of these dimers may be quenched in an environment that contains a significant population of organic ligands. The ionization and dissociation energies of the dimer are of same order which prevents the observation of the dimer in the ion mass spectroscopy. The analysis of the vibrational frequencies show the lowest-energy structure to be dynamically stable. We also present infrared absorption and Raman scattering spectra of the Ti_8C_12 dimer.

cond-mat.mtrl-sci↗

arXiv2002

Vibrationally induced fourth-order magnetic anisotropy and tunnel splittings in Mn_{12}

From density-functional-theory (DFT) based methods we calculate the vibrational spectrum of the Mn_{12}O_{12}(COOH)_{16}(H_2 O)_4 molecular magnet. Calculated infrared intensities are in accord with experimental studies. There have been no ab initio attempts at determining which interactions account for the fourth-order anisotropy. We show that vibrationally induced distortions of the molecule contribute to the fourth-order anisotropy Hamiltonian and that the magnitude and sign of the effect (-6.2 K) is in good agreement with all experiments. Vibrationally induced tunnel splittings in isotopically pure and natural samples are predicted.

cond-mat.mtrl-sci↗

arXiv2002

Magnetic ordering, electronic structure and magnetic anisotropy energy in the high-spin Mn$_{10}$ single molecule magnet

We report the electronic structure and magnetic ordering of the single molecule magnet [Mn$_{10}$O$_{4}$(2,2'-biphenoxide)$_{4}$Br$_{12}$]$^{4-}$ based on first-principles all-electron density-functional calculations. We find that two of the ten core Mn atoms are coupled antiferromagnetically to the remaining eight, resulting in a ferrimagnetic ground state with total spin S=13. The calculated magnetic anisotropy barrier is found to be 9 K in good agreement with experiment. The presence of the Br anions impact the electronic structure and therefore the magnetic properties of the 10 Mn atoms. However, the electric field due to the negative charges has no significant effect on the magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2001

The Hamiltonian of the V$_{15}$ Spin System from first-principles Density-Functional Calculations

We report first-principles all-electron density-functional based studies of the electronic structure, magnetic ordering and anisotropy for the V$_{15}$ molecular magnet. From these calculations, we determine a Heisenberg Hamiltonian with four antiferromagnetic and one {\em ferromagnetic} coupling. We perform direct diagonalization to determine the temperature dependence of the susceptibility. This Hamiltonian reproduces the experimentally observed spin $S$=1/2 ground state and low-lying $S$=3/2 excited state. A small anisotropy term is necessary to account for the temperature independent part of the magnetization curve.

cond-mat.mtrl-sci↗

arXiv1994

Density-Functional-Based Determination of the CH3-CH4 Hydrogen Exchange Reaction Barrier

Due to the overbinding that is inherent in existing {\em local} approximations to the density-functional formalism, certain reaction energies have not been accessible. Since the generalized gradient approximation significantly decreases the overbinding, prospects for density-functional-based reaction dynamics are promising. Results on the generalized-gradient based determination of the CH3-CH4 hydrogen exchange reaction are presented. Including all Born-Oppenheimer effects an energy barrier of 9.5 kcal/Mole is found which is a very significant improvement over the local-density approximation.

mtrl-th↗

arXiv1993

Electronic Structure of Superconducting Ba6c60

We report the results of first-principles electronic-structure calculations for superconducting Ba6C60. Unlike the A3C60 superconductors, this new compound shows strong Ba-C hybridization in the valence and conduction regions, mixed covalent/ionic bonding character, partial charge transfer, and insulating zero-gap band structure.

cond-mat↗