Hyperfine interactions for small systems including transition-metal elements using self-interaction corrected density-functional theory.

The interactions between the electronic magnetic moment and the nuclear spin moment, i.e., magnetic hyperfine (HF) interactions, play an important role in understanding electronic properties of magnetic systems and in realizing platforms for quantum information science applications. We investigate the HF interactions for atomic systems and small molecules, including Ti or Mn, by using Fermi-Löwdin orbital (FLO) based self-interaction corrected (SIC) density-functional theory. We calculate the Fermi contact (FC) and spin-dipole terms for the systems within the local density approximation (LDA) in the FLO-SIC method and compare them with the corresponding values without SIC within the LDA and generalized-gradient approximation (GGA), as well as experimental data. For the moderately heavy atomic systems (atomic number Z ≤ 25), we find that the mean absolute error of the FLO-SIC FC term is about 27 MHz (percentage error is 6.4%), while that of the LDA and GGA results is almost double that. Therefore, in this case, the FLO-SIC results are in better agreement with the experimental data. For the non-transition-metal molecules, the FLO-SIC FC term has the mean absolute error of 68 MHz, which is comparable to both the LDA and GGA results without SIC. For the seven transition-metal-based molecules, the FLO-SIC mean absolute error is 59 MHz, whereas the corresponding LDA and GGA errors are 101 and 82 MHz, respectively. Therefore, for the transition-metal-based molecules, the FLO-SIC FC term agrees better with experiment than the LDA and GGA results. We observe that the FC term from the FLO-SIC calculation is not necessarily larger than that from the LDA or GGA for all the considered systems due to the core spin polarization, in contrast to the expectation that SIC would increase the spin density near atomic nuclei, leading to larger FC terms.

Electronic structure of mononuclear Cu-based molecule from density-functional theory with self-interaction correction.

We investigate the electronic structure of a planar mononuclear Cu-based molecule [Cu(C6H4S2)2]z in two oxidation states (z = -2, -1) using density-functional theory (DFT) with Fermi-Löwdin orbital (FLO) self-interaction correction (SIC). The dianionic Cu-based molecule was proposed to be a promising qubit candidate. Self-interaction error within approximate DFT functionals renders severe delocalization of electron and spin densities arising from 3d orbitals. The FLO-SIC method relies on optimization of Fermi-Löwdin orbital descriptors (FODs) with which localized occupied orbitals are constructed to create SIC potentials. Starting with many initial sets of FODs, we employ a frozen-density loop algorithm within the FLO-SIC method to study the Cu-based molecule. We find that the electronic structure of the molecule remains unchanged despite somewhat different final FOD configurations. In the dianionic state (spin S = 1/2), FLO-SIC spin density originates from the Cu d and S p orbitals with an approximate ratio of 2:1, in quantitative agreement with multireference calculations, while in the case of SIC-free DFT, the orbital ratio is reversed. Overall, FLO-SIC lowers the energies of the occupied orbitals and, in particular, the 3d orbitals unhybridized with the ligands significantly, which substantially increases the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) compared to SIC-free DFT results. The FLO-SIC HOMO-LUMO gap of the dianionic state is larger than that of the monoanionic state, which is consistent with experiment. Our results suggest a positive outlook of the FLO-SIC method in the description of magnetic exchange coupling within 3d-element-based systems.

Icosahedral to double-icosahedral shape transition of copper clusters.

The lowest-energy isomers of Cu(N) clusters for N = 20-30 are identified using an unbiased search algorithm and density functional theory calculations. The low-energy structures over this size range are dominated by those based on a 13-atom icosahedral (I(h)) core and a 19-atom double icosahedron (DI(h)) core. A transition in the ground-state isomers from I(h)-based to DI(h)-based structures is predicted overt N = 21-23. We discuss this transition in the broader context of the growth pattern for Cu(N) over N = 2-30 that features regions of gradual evolution in which atoms successively add to the cluster surface, separated by sudden changes to a different structural organization and more compact shape. These transitions result from a competition between interatomic bonding energy and surface energy. The implications of this growth pattern for the further evolution of copper from microstructure to bulk are discussed.

Theoretical investigation of adsorption of molecular oxygen on small copper clusters.

Adsorption of molecular oxygen on Cu(N) (N = 2-10) clusters is investigated using density functional theory under the generalized gradient approximation of Perdew-Burke-Ernzerhof. An extensive structure search is performed to identify low-energy conformations of Cu(N)O(2) complexes. Optimal adsorption sites are assigned for low-energy isomers of the clusters. Among these are some new arrangements unidentified heretofore. Distinct size dependences are noted for the ground state Cu(N)O(2) complexes in stability, adsorption energy, Cu-O(2) bond strength, and other characteristic quantities. Cu(N)O(2) with odd-N tend to have larger adsorption energies than their even-N neighbors, with the exception of Cu(6)O(2), which has a relatively large adsorption energy resulting from the adsorption-induced 2D-to-3D structural transition in Cu(6). The energetically preferred spin-multiplicity of all the odd-N Cu(N)O(2) complexes is doublet; it is triplet for N = 2 and 4 and singlet for N = 6, 8, and 10.

Structural growth behavior and polarizability of Cd(n)Te(n) (n=1-14) clusters.

The lowest-energy structures of Cd(n)Te(n) (n=1-14) clusters have been studied by an unbiased simulated annealing search using first-principles molecular dynamics along with local optimization of "handmade" structures using density functional theory. After n>or=6, three-dimensional cage geometries are the lowest-energy configurations. Two families of low-lying structures, hollow cages, and endohedral or core-shell cages are found. The endohedral cages begin to appear from n=10, and they become more energetically preferred than the hollow cages for n>or=12. Cd(13)Te(13) with the core-shell cage structure is particularly stable. At the same size, the hollow cages possess smaller dipole moments and larger polarizabilities than the endohedral ones. The polarizabilities for the three-dimensional cage structures are insensitive to cluster size. The anisotropies in the polarizabilities mirror the anisotropies in cluster shapes.