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Tunable interactions in colloids, induced by rotating electric or magnetic fields, provide a flexible and promising tool for self-assembly of soft materials, as well as for fundamental particle-resolved studies of phase transitions and other generic phenomena in condensed matter. In the case of two-dimensional systems and the in-plane rotating fields, the interactions are known to have a long-range (dipolar) attraction and an expressed three-body part at short distances, but still remain poorly understood. Here, we show that the interactions and polarization mechanisms governing the tunable interactions can be described, calculated, and analyzed in detail with the diagrammatic method we proposed. The diagrams yield a clear illustration of different polarization processes contributing to the Keesom, Debye, London, self, and external energies, classified in colloids similarly to intermolecular interactions. The real tunable interactions, obtained with the boundary element method, can be simply and accurately interpolated with the set of basis of the diagrams attributed to different physically clear polarization processes. Calculation of large-distance behavior and interpolation of the many-body interactions (and analysis of the leading mechanisms contributing to them) excellently illustrate that the diagrammatic method provides deep insights into the nature of tunable interactions. The method can be generalized for multicomponent systems, suspensions of particles with a composite structure and a complicated shape. The results provide significant advance in theoretical methods for detailed analysis of tunable interactions in colloids and, therefore, the method is of broad interest in condensed matter, chemical physics, physical chemistry, materials science, and soft matter. [ABSTRACT FROM AUTHOR]

We present an approach to perform origin invariant optical rotation calculations in the length dipole gauge without recourse to London atomic orbitals, called origin invariant length gauge [LG(OI)]. The LG(OI) approach works with any approximate wave function or density functional method, but here we focus on the implementation with the coupled cluster (CC) with single and double excitations method because of the lack of production-level alternatives. Preliminary numerical tests show the efficacy of the LG(OI) procedure and indicate that putting the origin in the center of mass of a molecule may not be an optimal choice for conventional CC-LG calculations. [ABSTRACT FROM AUTHOR]

We present an origin-invariant approach to compute the full optical rotation tensor (Buckingham/Dunn tensor) in the length dipole gauge without recourse to London atomic orbitals, called LG(OI). The LG(OI) approach is simpler and less computationally demanding than the more common length gauge (LG)-London and modified velocity gauge (MVG) approaches, and it can be used with any approximate wave function or density functional method. We report an implementation at the coupled cluster with single and double excitations level (CCSD), for which we present the first simulations of the origin-invariant Buckingham/Dunn tensor in the LG. We compare LG(OI) and MVG results on a series of 22 organic molecules, showing good linear correlation between the approaches, although for small tensor elements, they provide values of opposite sign. We also attempt to decouple the effects of electron correlation and basis set incompleteness on the choice of gauge for specific rotation calculations on simple test systems. The simulations show a smooth convergence of the LG(OI) and MVG results with the basis set size toward the complete basis set limit. However, these preliminary results indicate that CCSD may not be close to a complete description of the electron correlation effects on this property even for small molecules and that basis set incompleteness may be a less important cause of discrepancy between choices of gauge than electron correlation incompleteness. [ABSTRACT FROM AUTHOR]

The dynamics of a molecule in a magnetic field is significantly different from its zero-field counterpart. One important difference in the presence of a field is the Lorentz force acting on the nuclei, which can be decomposed as the sum of the bare nuclear Lorentz force and a screening force due to the electrons. This screening force is calculated from the Berry curvature and can change the dynamics qualitatively. It is therefore important to include the contributions from the Berry curvature in molecular dynamics simulations in a magnetic field. In this work, we present a scheme for calculating the Berry curvature numerically using a finite-difference technique, addressing challenges related to the arbitrary global phase of the wave function. The Berry curvature is calculated as a function of bond distance for H2 at the restricted and unrestricted Hartree–Fock levels of theory and for CH+ as a function of the magnetic field strength at the restricted Hartree–Fock level of theory. The calculations are carried out using basis sets of contracted Gaussian functions equipped with London phase factors (London orbitals) to ensure gauge-origin invariance. In this paper, we also interpret the Berry curvature in terms of atomic charges and discuss its convergence in basis sets with and without London phase factors. The calculation of the Berry curvature allows for its inclusion in ab initio molecular dynamics simulations in a magnetic field. [ABSTRACT FROM AUTHOR]

We show that two positrons can form a chemical bond between two otherwise repelling ions, similar to what happens to two hydrogen atoms forming a hydrogen molecule. Two positronium hydride atoms (PsH) can form the stable species (PsH)2 when the two coupled positrons have opposite spins, while they form an antibonding state if they have the same spin. This is completely analogous to the landmark description by Heitler and London [Z. Phys. 44, 455 (1927)] on the formation of a chemical bond in the hydrogen molecule coupling two electrons with opposite spins. This is the first time two positrons are shown to behave like two electrons in ordinary matter, enlarging the definition of what is a chemical bond dating back to Lewis [J. Am. Chem. Soc. 38, 762 (1916)]. We suggest a few experimental routes to form and detect such a peculiar molecule. [ABSTRACT FROM AUTHOR]

Dispersion interactions are one of the components of van der Waals (vdW) forces that play a key role in the understanding of intermolecular interactions in many physical, chemical, and biological processes. The theory of dispersion forces was developed by London in the early years of quantum mechanics. However, it was only in the 1960s that it was recognized that for molecules lacking an inversion center, such as chiral and helical molecules, there are chirality-sensitive corrections to the dispersion forces proportional to the rotatory power known from the theory of circular dichroism and with the same distance scaling law R−6 as the London energy. The discovery of the chirality-induced spin selectivity effect in recent years has led to an additional twist in the study of chiral molecular systems, showing a close relation between spin and molecular geometry. Motivated by it, we propose in this investigation to describe the mutual induction of charge and spin-density fluctuations in a pair A–B of chiral molecules by a simple physical model. The model assumes that the same fluctuating electric fields responsible for vdW forces can induce a magnetic response via a Rashba-like term so that a spin–orbit field acting on molecule B is generated by the electric field arising from charge density fluctuations in molecule A (and vice versa). Within a second-order perturbative approach, these contributions manifest as an effective intermolecular exchange interaction. Although expected to be weaker than the standard London forces, these interactions display the same R−6 distance scaling. [ABSTRACT FROM AUTHOR]

Molecular rotations and vibrations have been extensively studied by chemists for decades, both experimentally using spectroscopic methods and theoretically with the help of quantum chemistry. However, the theoretical investigation of molecular rotations and vibrations in strong magnetic fields requires computationally more demanding tools. As such, proper calculations of rotational and vibrational spectra were not feasible up until very recently. In this work, we present rotational and vibrational spectra for two small linear molecules, H2 and LiH, in strong magnetic fields. By treating the nuclei as classical particles, trajectories for rotations and vibrations are simulated from ab initio molecular dynamics. Born–Oppenheimer potential energy surfaces are calculated at the Hartree–Fock and MP2 levels of theory using London atomic orbitals to ensure gauge origin invariance. For the calculation of nuclear trajectories, a highly efficient Tajima propagator is introduced, incorporating the Berry curvature tensor accounting for the screening of nuclear charges. [ABSTRACT FROM AUTHOR]

Routinely assessing the stability of molecular crystals with high accuracy remains an open challenge in the computational sciences. The many-body expansion decomposes computation of the crystal lattice energy into an embarrassingly parallel collection of computations over molecular dimers, trimers, and so forth, making quantum chemistry techniques tractable for many crystals of small organic molecules. By examining the range-dependence of different types of energetic contributions to the crystal lattice energy, we can glean qualitative understanding of solid-state intermolecular interactions as well as practical, exploitable reductions in the number of computations required for accurate energies. Here, we assess the range-dependent character of two-body interactions of 24 small organic molecular crystals by using the physically interpretable components from symmetry-adapted perturbation theory (electrostatics, exchange-repulsion, induction/polarization, and London dispersion). We also examine correlations between the convergence rates of electrostatics and London dispersion terms with molecular dipole moments and polarizabilities, to provide guidance for estimating convergence rates in other molecular crystals. [ABSTRACT FROM AUTHOR]