Small semiconductor silver and gold sulfide clusters $({\mathrm{Ag}}_{\mathrm{2}}\mathrm{S}{)}_n$ and $({\mathrm{Au}}_{\mathrm{2}}\mathrm{S}{)}_n,$ $\text{n=1,2,}$ are studied by valence ab initio calculations with the inclusion of electron correlation at the second-order perturbation theory (MP2) and coupled-cluster [CCSD and CCSD(T)] levels. Various relativistic and nonrelativistic pseudopotentials are employed to describe the effects of core electrons. Correlation and relativistic effects are essential in determining the geometry and relative stability of monomer and dimer structures. Relativistic effects result in a notable decrease in the calculated interatomic distances, which is especially significant in the case of gold sulfide structures (up to 10%). Correlation effects markedly increase the stability of compact structures with an increased number of relatively short M…M contacts (M…M distances of about 280–330 pm). Excluding the correlation of lower-lying valence orbitals (sulfur $\text{3s}$ and silver $\text{4d}$ or gold $\text{5d)}$ results in completely opposite predictions. This fact suggests that the effects of $\mathrm{d–d}$ and d–outer valence (metal $\mathrm{ns}$ and sulfur $\text{3p)}$ electron correlation give rise to attractive short-range interactions of intramolecular van der Waals type, which determine the increased stability of more compact cluster structures. However, large-core pseudopotentials strongly exaggerate this effect in the case of gold and give results rather different from those obtained with more valid and accurate small-core pseudopotentials. It is shown that the reason for this deficiency lies in the nature of pseudopotentials themselves rather than in basis-set shortcomings. The atomization and dissociation energies, equilibrium geometrical parameters, dipole moments, and Mulliken populations are calculated and discussed.

Stability region for the only stable SrFeO_2.5 phase is drawn as a function of temperature and oxygen gas partial pressure.

The crystalline materials with nonlinear optical (NLO) properties are critically important for several technological applications, including nanophotonic and second harmonic generation devices. Urea is often considered to be a standard NLO material, due to the combination of non-centrosymmetric crystal packing and capacity for intramolecular charge transfer. Various approaches to crystal engineering of non-centrosymmetric molecular materials were reported in the literature. Here we propose using global lattice energy minimization to predict the crystal packing from the first principles. We developed a methodology that includes the following: (1) parameter derivation for polarizable force field AMOEBA; (2) local minimizations of crystal structures with these parameters, combined with the evolutionary algorithm for a global minimum search, implemented in program USPEX; (3) filtering out duplicate polymorphs produced; (4) reoptimization and final ranking based on density functional theory (DFT) with many-body dispersion (MBD) correction; and (5) prediction of the second-order susceptibility tensor by finite field approach. This methodology was applied to predict virtual urea polymorphs. After filtering based on packing similarity, only two distinct packing modes were predicted: one experimental and one hypothetical. DFT + MBD ranking established non-centrosymmetric crystal packing as the global minimum, in agreement with the experiment. Finite field approach was used to predict nonlinear susceptibility, and H-bonding was found to account for a 2.5-fold increase in molecular hyperpolarizability to the bulk value.