Search results for "Clusters"
showing 10 items of 1274 documents
Insights on the coupling between vibronically active molecular vibrations and lattice phonons in molecular nanomagnets
2021
Spin-lattice relaxation is a key open problem to understand the spin dynamics of single-molecule magnets and molecular spin qubits. While modelling the coupling between spin states and local vibrations allows to determine the more relevant molecular vibrations for spin relaxation, this is not sufficient to explain how energy is dissipated towards the thermal bath. Herein, we employ a simple and efficient model to examine the coupling of local vibrational modes with long-wavelength longitudinal and transverse phonons in the clock-like spin qubit [Ho(W$_5$O$_{18}$)$_2$]$^{9-}$. We find that in crystals of this polyoxometalate the vibrational mode previously found to be vibronically active at …
Analytic energy gradients in closed-shell coupled-cluster theory with spin-orbit coupling
2008
Gradients in closed-shell coupled-cluster (CC) theory with spin-orbit coupling included in the post Hartree-Fock treatment have been implemented at the CC singles and doubles (CCSD) level and at the CCSD level augmented by a perturbative treatment of triple excitations [CCSD(T)]. The additional computational effort required in analytic energy-gradient calculations is roughly the same as that for ground-state energy calculations in the case of CCSD, and it is about twice in the case of CCSD(T) calculations. The structures, harmonic frequencies, and dipole moments of some heavy-element compounds have been calculated using the present analytic energy-gradient techniques including spin-orbit co…
Odd-even staggering in simple models of metal clusters
1994
The odd-even staggering of free-electron metal clusters is studied using several simple models: Noninter-acting electrons in a rectangular box, triaxial harmonic oscillator, and Huckel model. Finite temperature effects are studied using the Monte Carlo method. All the models show qualitatively similar odd-even staggering. In the ground state the HOMO-LUMO gap is larger than the neighbouring energy gaps. The reduction of the odd-even staggering due to exchange and correlation is studied using the local-spin-density approximation.
Explicitly correlated coupled-cluster theory using cusp conditions. I. Perturbation analysis of coupled-cluster singles and doubles (CCSD-F12)
2010
Geminal functions based on Slater-type correlation factors and fixed expansion coefficients, determined by cusp conditions, have in recent years been forwarded as an efficient and numerically stable method for introducing explicit electron correlation into coupled-cluster theory. In this work, we analyze the equations of explicitly correlated coupled-cluster singles and doubles (CCSD-F12) theory and introduce an ordering scheme based on perturbation theory which can be used to characterize and understand the various approximations found in the literature. Numerical results for a test set of 29 molecules support our analysis and give additional insight. In particular, our results help ration…
Explicitly correlated coupled-cluster theory using cusp conditions. II. Treatment of connected triple excitations.
2010
The coupled-cluster singles and doubles method augmented with single Slater-type correlation factors (CCSD-F12) determined by the cusp conditions (also denoted as SP ansatz) yields results close to the basis set limit with only small overhead compared to conventional CCSD. Quantitative calculations on many-electron systems, however, require to include the effect of connected triple excitations at least. In this contribution, the recently proposed [A. Köhn, J. Chem. Phys. 130, 131101 (2009)] extended SP ansatz and its application to the noniterative triples correction CCSD(T) is reviewed. The approach allows to include explicit correlation into connected triple excitations without introducin…
Emitter-site specificity of hard x-ray photoelectron Kikuchi-diffraction
2020
New journal of physics 22(10), 103002 (1-13) (2020). doi:10.1088/1367-2630/abb68b
Dipole surface plasmon in large K N + clusters
1993
The dipole surface plasmon forK N + clusters is analyzed using the RPA sum-rule technique within a semiclassical Density Functional Theory and the spherical jellium model. The theoretical frequencies are blue shifted as compared to the experimental ones. The discrepancies between theory and experiment are reduced when considering non-local energy contributions in the density functional and phenomenologically including atomic lattice effects by means of an electron effective mass and a static dielectric constant.
Finite perturbation calculations for the static dipole polarizabilities of the first-row atoms
1976
Static dipole polarizabilities are calculated from self-consistent-field and highly correlated wave functions for the ground states of the atoms Li through Ne. The correlation contributions to the polarizabilities are found to vary between -16% for Be and + 14% for F. The polarizabilities as obtained from the coupled-electron-pair approximation are expected to be accurate to about 2%.
Optical response and shapes of charged sodium clusters; an analogue of the nuclear giant dipole response
1995
Collective vibrations of de-localized electrons against the positive charged ionic background in sodium clusters (plasmon resonances) and the collective vibrations of protons against neutrons in nuclei (giant dipole resonances) have several features in common. However, fundamental differences exist due to differences in the two media; the nucleus is a quantum liquid whereas the metallic medium is more like a plasma with classical positive ions and quantized electrons. The similarities and differences are illustrated by results from optical response measurements for charged sodium clusters with 14 to 48 atoms.
Variational Study of3HeDroplets
1999
We report variational calculations of energies of ${}^{3}{\mathrm{He}}_{N}$ droplets ( $20\ensuremath{\le}N\ensuremath{\le}40$), using Aziz atom-atom interactions. The trial wave function has a simple structure, combining two- and three-body correlation functions coming from a translationally invariant configuration-interaction description, superimposed to a Jastrow-type correlated wave function with backflow. We find that the smallest bound drop has $N\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}35$ atoms, and that for each $N$ the minimum energy states have the highest spin values.