Non-expanded dispersion energies and damping functions for Ar2 and Li2

Physical mechanisms of intermolecular interactions from symmetry-adapted perturbation theory

Symmetry-adapted perturbation theory (SAPT) is a method for computational studies of noncovalent interactions between molecules. This method will be discussed here from the perspective of establishing the paradigm for understanding mechanisms of intermolecular interactions. SAPT interaction energies are obtained as sums of several contributions. Each contribution possesses a clear physical interpretation as it results from some specific physical process. It also exhibits a specific dependence on the intermolecular separation R. The four major contributions are the electrostatic, induction, dispersion, and exchange energies, each due to a different mechanism, valid at any R. In addition, at large R, SAPT interaction energies are seamlessly connected with the corresponding terms in the asymptotic multipole expansion of interaction energy in inverse powers of R. Since such expansion explicitly depends on monomers' multipole moments and polarizabilities, this connection provides additional insights by rigorously relating interaction energies to monomers' properties.

Non-covalent interactions from a Quantum Chemical Topology perspective

About half a century after its little-known beginnings, the quantum topological approach called QTAIM has grown into a widespread, but still not mainstream, methodology of interpretational quantum chemistry. Although often confused in textbooks with yet another population analysis, be it perhaps an elegant but somewhat esoteric one, QTAIM has been enriched with about a dozen other research areas sharing its main mathematical language, such as Interacting Quantum Atoms (IQA) or Electron Localisation Function (ELF), to form an overarching approach called Quantum Chemical Topology (QCT). Instead of reviewing the latter's role in understanding non-covalent interactions, we propose a number of ideas emerging from the full consequences of the space-filling nature of topological atoms, and discuss how they (will) impact on interatomic interactions, including non-covalent ones. The architecture of a force field called FFLUX, which is based on these ideas, is outlined. A new method called Relative Energy Gradient (REG) is put forward, which is able, by computation, to detect which fragments of a given molecular assembly govern the energetic behaviour of this whole assembly. This method can offer insight into the typical balance of competing atomic energies both in covalent and non-covalent case studies. A brief discussion on so-called bond critical points is given, highlighting concerns about their meaning, mainly in the arena of non-covalent interactions.

Quadrupole and octupole polarizabilities for the ground states of lithiumlike systems from Z=3 to 20

The quadrupole and octupole polarizabilities for the ground states of lithiumlike systems from Z=3 to 20 are calculated with the full-core plus correlation method. For the neutral lithium atom, the typical patterns of convergence of the quadrupole and octupole polarizabilities are analyzed. The calculated quadrupole and octupole polarizabilities of the ground state for lithium atom are compared with the previous theoretical results obtained by other methods; our predictions agree with the most accurate reports in the literature very well. For lithiumlike ions, our prediction may provide valuable reference data for other accurate theoretical calculations in future.

Dispersion energy from density-functional theory description of monomers

A method is proposed for calculations of dispersion energy at finite intermonomer separations. It uses a generalized Casimir-Polder formula evaluated with dynamic density susceptibilities provided by time-dependent density-functional theory. The method recovers the dispersion energies of He, Ne, and H2O dimers to within 3% or better. Since the computational effort of the new algorithm scales approximately as the third power of system size, the method is much more efficient than standard wave-function methods capable of predicting the dispersion energy at a similarly high level of accuracy.