We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of interatomic C6 coefficients, derived from the electron density of a molecule or solid and accurate reference data for the free atoms. The mean absolute error in the C6 coefficients is 5.5% when compared to accurate experimental values for 1225 intermolecular pairs, irrespective of the employed exchangecorrelation functional. We show that the effective atomic C6 coefficients depend strongly on the bonding environment of an atom in a molecule. Finally, we analyze the van der Waals radii and the damping function in the C6R � 6 correction method for density-functional theory calculations.

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Accurate molecular van der Waals interactions from ground-state electron density and free-atom reference data

We have constructed an optical clock with a fractional frequency inaccuracy of 8.6x10{sup -18}, based on quantum logic spectroscopy of an Al{sup +} ion. A simultaneously trapped Mg{sup +} ion serves to sympathetically laser cool the Al{sup +} ion and detect its quantum state. The frequency of the {sup 1}S{sub 0}reversible{sup 3}P{sub 0} clock transition is compared to that of a previously constructed Al{sup +} optical clock with a statistical measurement uncertainty of 7.0x10{sup -18}. The two clocks exhibit a relative stability of 2.8x10{sup -15}tau{sup -1/2}, and a fractional frequency difference of -1.8x10{sup -17}, consistent with the accuracy limit of the older clock.

/pdf/frequency-comparison-of-two-high-accuracy-al-optical-clocks-i1j872haiw.pdf

Frequency Comparison of Two High-Accuracy Al+ Optical Clocks

https://journals.iucr.org/m/issues/2017/05/00/lc5090/lc5090.pdf

CrystalExplorer model energies and energy frameworks: extension to metal coordination compounds, organic salts, solvates and open-shell systems

Atomic Spectra Database

Positron annihilation spectroscopy is particularly suitable for studying vacancy-type defects in semiconductors. Combining state-of-the-art experimental and theoretical methods allows for detailed identification of the defects and their chemical surroundings. Also charge states and defect levels in the band gap are accessible. In this review the main experimental and theoretical analysis techniques are described. The usage of these methods is illustrated through examples in technologically important elemental and compound semiconductors. Future challenges include the analysis of noncrystalline materials and of transient defect-related phenomena.

/pdf/defect-identification-in-semiconductors-with-positron-kn4qxl00fh.pdf

Defect identification in semiconductors with positron annihilation: Experiment and theory

Atomic polarization phenomena impinge upon a number of areas and processes in physics. The dielectric constant and refractive index of any gas are examples of macroscopic properties that are largely determined by the dipole polarizability. When it comes to microscopic phenomena, the existence of alkaline-earth anions and the recently discovered ability of positrons to bind to many atoms are predominantly due to the polarization interaction. An imperfect knowledge of atomic polarizabilities is presently looming as the largest source of uncertainty in the new generation of optical frequency standards. Accurate polarizabilities for the group I and II atoms and ions of the periodic table have recently become available by a variety of techniques. These include refined many-body perturbation theory and coupled-cluster calculations sometimes combined with precise experimental data for selected transitions, microwave spectroscopy of Rydberg atoms and ions, refractive index measurements in microwave cavities, ab initio calculations of atomic structures using explicitly correlated wavefunctions, interferometry with atom beams and velocity changes of laser cooled atoms induced by an electric field. This review examines existing theoretical methods of determining atomic and ionic polarizabilities, and discusses their relevance to various applications with particular emphasis on cold-atom physics and the metrology of atomic frequency standards.

https://iopscience.iop.org/article/10.1088/0953-4075/43/20/202001/pdf

Theory and applications of atomic and ionic polarizabilities

The variational method complemented with the use of explicitly correlated Gaussian basis functions is one of the most powerful approaches currently used for calculating the properties of few-body systems. Despite its conceptual simplicity, the method offers great flexibility, high accuracy, and can be used to study diverse quantum systems, ranging from small atoms and molecules to light nuclei, hadrons, quantum dots, and Efimov systems. The basic theoretical foundations are discussed, recent advances in the applications of explicitly correlated Gaussians in physics and chemistry are reviewed, and the strengths and weaknesses of the explicitly correlated Gaussians approach are compared with other few-body techniques.

Theory and application of explicitly correlated Gaussians

Recent research has shown that there are a number of atoms and atomic ions that can bind a positron. The number of atoms known to be capable of binding a positron has expanded enormously in recent years, with Li, He(3Se), Be, Na, Mg, Ca, Cu, Zn, Sr, Ag and Cd all capable of binding a positron. The structure of these systems is largely determined by the competition between the positron and the nucleus to bind the loosely bound valence electrons. Some systems, such as e+Li and e+Na, can be best described as a Ps cluster orbiting a charged Li+ or Na+ core, while others such as e+Be consist of a positron orbiting a polarized Be atom. In addition, a number of atoms (Li, C, O, F, Na, Cl, K, Cl, Cu, Br) can bind positronium and a few systems capable of binding two positrons have also been identified. These positron-binding systems decay by electron-positron annihilation with the annihilation rate for e+A systems largely determined by the parent atom ionization potential.

https://iopscience.iop.org/article/10.1088/0953-4075/35/13/201/pdf

Positron and positronium binding to atoms

The van der Waals coefficients, ${C}_{6},{C}_{8},$ and ${C}_{10}$ for the alkali-metal (Li, Na, K, and Rb) and alkaline-earth-metal (Be, Mg, Ca, and Sr) atoms are estimated by a combination of ab initio and semiempirical methods. Polarizabilities and atom-wall coefficients are given as a diagnostic check, and the lowest order nonadiabatic dispersion coefficient, ${D}_{8}$ and the three-body coefficient, ${C}_{9}$ are also presented. The dispersion coefficients are in agreement with the available relativistic many-body perturbation theory calculations. The contribution from the core was included by using constrained sum rules involving the core polarizability and Hartree-Fock expectation values to estimate the f-value distribution.

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Semiempirical calculation of van der Waals coefficients for alkali-metal and alkaline-earth-metal atoms

The structures of a number of exotic atoms with an attached positron or positronium atom are studied using a large-scale variational expansion in terms of a basis of explicitly correlated Gaussian functions. The binding energies and annihilation rates for seven exotic species with electronically stable ground states, namely HPs, , LiPs, , , NaPs and have been predicted. The binding energy for HPs, 0.038 1944 Hartree, is the largest attained so far. Two of the species, and , with approximate binding energies of 0.0024 and 0.0005 Hartree respectively, are seen to have structures best described as a positronium atom orbiting a residual or positively charged core. The atom with an approximate binding energy of 0.0028 Hartree is best characterized as a positron orbiting a polarized Be core. The binding energy of the ground state, 0.014 Hartree, is larger than that of any other positronic atom (a neutral atom with an attached positron). The LiPs and NaPs atoms, with approximate binding energies of 0.012 and 0.0072 Hartree respectively, have structures similar to HPs although the binding energies are smaller and the valence electrons and the positron are found at larger distances from the nucleus.

http://iopscience.iop.org/article/10.1088/0953-4075/31/17/019/pdf

Jim Mitroy

Papers

Theory and applications of atomic and ionic polarizabilities

Theory and application of explicitly correlated Gaussians

Positron and positronium binding to atoms

Semiempirical calculation of van der Waals coefficients for alkali-metal and alkaline-earth-metal atoms

The structure of exotic atoms containing positrons and positronium