Search results for " orbita"
showing 10 items of 447 documents
First-principles LCAO study of phonons in NiWO4
2011
Abstract The electronic, structural and phonon properties of antiferromagnetic wolframite-type NiWO4 have been studied using first-principles spin-polarized LCAO calculations based on the hybrid Hartree-Fock (HF)/density functional (DFT) scheme. The influence of different percentages of HF contribution, i.e. different correlation strength, on the structure and phonon frequencies has been investigated and compared with the available experimental data.
Orbital origin and matrix element effects in the Ag/Si(111)-()R30° Fermi surface
2007
The Fermi surface (FS) of the Ag/Si(1 1 1)-3×3 reconstruction with an excess of Ag has been mapped by angle resolved photoemission spectroscopy with polarized light in a wide region of the reciprocal space and with different detection geometries. In contrast to previous results, a strong polarization dependence is observed. Applying the dipole selection rules, it is found that the surface state at the Fermi level, S1 state, has odd symmetry with respect to the mirror plane of the honeycomb-chained triangle structure, indicating that it is mainly derived from Ag 5px and 5py orbitals. This conclusion is revised in the new frame of a inequivalent-triangle structure for the Ag/Si(1 1 1)-3×3 at …
Quantum chemical modelling of "green" luminescence in ABO $ \mathsf {_3}$ perovskites
2002
The origin of the intrinsic excitonic (“green”) luminescence in ABO3 perovskites remains a hot topic over the last quarter of a century. We suggest as a theoretical interpretation for the “green” luminescence in these crystals, the recombination of electron and hole polarons forming a charge transfer vibronic exciton. In order to check quantitatively the proposed model, we performed quantum chemical calculations using the Intermediate Neglect of Differential Overlap (INDO) method combined with the periodic defect model. The luminescence energies calculated for four perovskite crystals are found to be in good agreement with experimental data.
Insights into the orbital invariance problem in state-specific multireference coupled cluster theory.
2010
In this communication we report the results of our studies on the orbital invariance properties of the state-specific multireference coupled cluster approach suggested by Mukherjee and co-workers (Mk-MRCC). In particular, we have gathered numerical evidence to show that even when the linear excitation manifold is modified in order to span the same space for each reference, the resulting method is not orbital invariant. In order to test this conjecture we have proposed a new truncation scheme (Mk-MRCCSDtq) which, in addition to full single and double excitations, contains partial triple and quadruple excitations. For a reference space generated by all possible combinations of two electrons i…
Perturbative treatment of spin-orbit-coupling within spin-free exact two-component theory using equation-of-motion coupled-cluster methods.
2018
A scheme is reported for the perturbative calculation of spin-orbit coupling (SOC) within the spin-free exact two-component theory in its one-electron variant (SFX2C-1e) in combination with the equation-of-motion coupled-cluster singles and doubles method. Benchmark calculations of the spin-orbit splittings in 2Π and 2P radicals show that the accurate inclusion of scalar-relativistic effects using the SFX2C-1e scheme extends the applicability of the perturbative treatment of SOC to molecules that contain heavy elements. The contributions from relaxation of the coupled-cluster amplitudes are shown to be relatively small; significant contributions from correlating the inner-core orbitals are …
Octupole states in Tl207 studied through β decay
2020
The β decay of 207Hg into the single-proton-hole nucleus 207Tl has been studied through γ-ray spectroscopy at the ISOLDE Decay Station (IDS) with the aim of identifying states resulting from coupling of the πs−11/2, πd−13/2, and πh−111/2 shell model orbitals to the collective octupole vibration. Twenty-two states were observed lying between 2.6 and 4.0 MeV, eleven of which were observed for the first time, and 78 new transitions were placed. Two octupole states (s1/2-coupled) are identified and three more states (d3/2-coupled) are tentatively assigned using spin-parity inferences, while further h11/2-coupled states may also have been observed for the first time. Comparisons are made with st…
Closed-shell coupled-cluster theory with spin-orbit coupling
2008
A two-component closed-shell coupled-cluster (CC) approach using relativistic effective core potentials with spin-orbit coupling included in the post-Hartree-Fock treatment is proposed and implemented at the CC singles and doubles (CCSD) level as well as at the CCSD level augmented by a perturbative treatment of triple excitations [CCSD(T)]. The latter invokes as an additional approximation the neglect of the occupied-occupied and virtual-virtual blocks of the spin-orbit coupling matrix in order to avoid the iterative N(7) steps in the treatment of triple excitations. The computational effort of the implemented two-component CC methods is about 10-15 times that of its corresponding nonrelat…
Exotic magnetism in the alkali sesquioxidesRb4O6andCs4O6
2009
Among the various alkali oxides the sesquioxides ${\text{Rb}}_{4}{\text{O}}_{6}$ and ${\text{Cs}}_{4}{\text{O}}_{6}$ are of special interest. Electronic-structure calculations using the local spin-density approximation predicted that ${\text{Rb}}_{4}{\text{O}}_{6}$ should be a half-metallic ferromagnet, which was later contradicted when an experimental investigation of the temperature-dependent magnetization of ${\text{Rb}}_{4}{\text{O}}_{6}$ showed a low-temperature magnetic transition and differences between zero-field-cooled and field-cooled measurements. Such behavior is known from spin glasses and frustrated systems. ${\text{Rb}}_{4}{\text{O}}_{6}$ and ${\text{Cs}}_{4}{\text{O}}_{6}$ c…
Diverging exchange force and form of the exact density matrix functional
2019
For translationally invariant one-band lattice models, we exploit the ab initio knowledge of the natural orbitals to simplify reduced density matrix functional theory (RDMFT). Striking underlying features are discovered: First, within each symmetry sector, the interaction functional $\mathcal{F}$ depends only on the natural occupation numbers $\bf{n}$. The respective sets $\mathcal{P}^1_N$ and $\mathcal{E}^1_N$ of pure and ensemble $N$-representable one-matrices coincide. Second, and most importantly, the exact functional is strongly shaped by the geometry of the polytope $\mathcal{E}^1_N \equiv \mathcal{P}^1_N $, described by linear constraints $D^{(j)}(\bf{n})\geq 0$. For smaller systems,…
Origin invariant calculation of optical rotation without recourse to London orbitals
2004
We propose an inherently origin invariant formulation of specific optical rotation obtained from the conventional velocity gauge expression by subtracting the static limit from the electric dipole-magnetic dipole polarizability: ããpâ,LâããÏâããpâ,LâããÏ-ããpâ,Lâãã0. London (gauge including) atomic orbitals are therefore not needed to ensure origin invariance and, consequently, this modified velocity gauge formulation may be employed in conjunction with variational and non-variational quantum chemical methods alike. Sample calculations on S-propylene oxide and 3,4- methylenedioxymethamphetamine using self-consistent field and coupled cluster wave functions…