Water is an ubiquitous liquid that has several exotic and anomalous properties. Despite its apparent simple chemical formula, its capability of forming a dynamic network of hydrogen bonds leads to a rich variety of physics. Here we study the vibrations of water using molecular dynamics simulations, mainly concentrating on the Raman and infrared spectroscopic signatures. We investigate the consequences of the temperature on the vibrational frequencies, and we enter the details of the hydrogen bonding coordination by using restrained simulations in order to gain quantitative insight on the dependence of the frequencies on the neighbouring molecules. Further we consider the differences due to the different methods of solving the electronic structure to evaluate the forces on the ions, and report results on the angular correlations, isotopic mixtures HOD in H2O/D2O and and the dielectric constants in water.

It was found recently that the liquid dynamics of several metals (Li, Zn, Ni, Fe, Tl, Pb) under pressure is characterized by transverse spectral functions containing an additional high-frequency peak. To rationalize the pressure dependence of the contributions from different propagating processes to transverse spectral functions in liquid metals, ab initio molecular dynamics simulations were performed for two typical liquid metals (Na and Al) in a wide range of pressures. The influence of density/pressure is investigated for Na by considering four pressures ranging from 15 to 147 GPa, while the temperature influence is considered for Al between 600 K in the deep supercooled liquid up to 1700 K well above the melting point at ambient pressure. Both temperature and density dependence of the spectra of collective excitations are analyzed with a focus on the appearance of a second high-frequency mode in the transverse spectra. A correspondence between spectra of transverse collective excitations and the peak positions of the Fourier-spectra of velocity autocorrelation functions (vibrational density of states) is found.

Structural transition induced by a local conformational change in biomolecules is formulated based on the generalized Langevin theory for the structural fluctuation of a molecule in solution, and the linear response theory, derived by Kim and Hirata in 2012. A chemical/mechanical change introduced at a moiety of biomolecules, such as an amino acid substitution or a structural change of a chromophore by the photo-excitation, is considered as a perturbation, and the rest of the protein as the reference system. The linear-response equation consists of two parts: a mechanical/chemical perturbation introduced at the moiety, and the variance-covariance matrix of the reference system that works as a response function. The physical meaning of the equation is transparent: the force exerted by atoms in the moiety induces the displacement in an atom of protein, which propagates through the variance-covariance matrix to cause a global conformational change in the molecule. A few examples of possible application of the theory, including those in industry, are suggested.

Although well described by mean-field theory in the thermodynamic limit, scaling has long been puzzling for finite systems in high dimensions. This raised questions about the efficacy of the renormalization group and foundational concepts such as universality, finite-size scaling and hyperscaling, until recently believed not to be applicable above the upper critical dimension. Significant theoretical progress has been made resolving these issues, and tested in numerous simulational studies of spin models. This progress rests upon superlinearity of correlation length, a notion that for a long time encountered resistance but is now broadly accepted. Percolation theory brings added complications such as proliferation of interpenetrating clusters in apparent conflict with suggestions coming from random-graph asymptotics and a dearth of reliable simulational guidance. Here we report on recent theoretical progress in percolation theory in the renormalization group framework in high dimensions that accommodates superlinear correlation and renders most of the above concepts mutually compatible under different boundary conditions. Results from numerical simulations for free and periodic boundary conditions which differentiate between previously competing theories are also presented. Although still fragmentary, these Monte Carlo results support the new framework which restores the renormalization group and foundational concepts on which it rests.

On January 2022 Dr. Orest Pizio turns his 70th anniversary being one of the founders of Condensed Matter Physics (CMP) journal and an actual member of its Editorial Board. Nowadays, CMP is an internationally well-known journal, recognized by the European Physical Society and covered by Web of Science Core Collection. The very first volume of Condensed Matter Physics published in 1993 contained contributions of Ukrainian researchers, mainly from Lviv. Orest edited the second volume and performed a pioneering job of inviting the researchers from around the world to submit their papers for publication in CMP.

In the present study we discuss the effect of variation in the number of monolayers n on the electronic and optical properties of superlattices (SLs) (ZnSe)n/(ZnTe)n. The total energies were calculated by the full-potential linear muffin-tin orbital (FP-LMTO) method, and the exchange-correlation energy was applied in the local density approximation (LDA). First, the calculations show a decrease in the derivative of bulk modulus and electronic bandgap with an increase in the number of monolayers n. Second, the radiation energies up to 15 eV, the dielectric function ε(ω), the refractive index n(ω), and the reflectivity R(ω) are studied. These calculations may be beneficial to understand the properties of short-period superlattices (ZnSe)n/(ZnTe)n.

In simulations of crystals, unlike liquids or gases, it may happen that the properties of the studied system depend not only on the volume of the simulation cell but also on its shape. For such cases it is desirable to change the shape of the box on the fly in the course of the simulation as it may not be known ahead of time which geometry fits the studied system best. In this work we derive an algorithm for this task based on the condition that the distribution of specific geometrical parameter observed in simulations at a constant volume matches that observed in the constant-pressure ensemble. The proposed algorithm is tested for the system of hard-core ellipses which makes lattices of different types depending on the asphericity parameter of the particle. It is shown that the performance of the algorithm critically depends on the range of the sampled geometrical parameter. If the range is narrow, the impact of the sampling method is minimal. If the range is large, inadequate sampling can lead to significant distortions of the relevant distribution functions and, as a consequence, errors in the estimates of free energy.

The spin-1/2 Heisenberg model is formulated in terms of a mean-field approximation (MFA) by using the matrix forms of spin operators Ŝx, Ŝy and Ŝz in three-dimensions. The considered Hamiltonian consists of bilinear exchange interaction parameters (Jx, Jy, Jz), Dzyaloshinskii-Moriya interactions (Δx, Δy, Δz) and external magnetic field components (Hx, Hy, Hz). The magnetization and its components are obtained in the MFA with the general anisotropic case with Jx ≠ Jy ≠ Jz for various values of coordination numbers q. Then, the thermal variations of magnetizations are investigated in detail to obtain the phase diagrams of the model for the isotropic case with Jx = Jy = Jz > 0. It is found that the model exhibits ferromagnetic, paramagnetic, random phase regions and an extra ferromagnetic phase at which the components of magnetizations present branching.

Dielectric permittivity was studied in ceramics of relaxor ferroelectric bismuth-sodium titanate Na0.5Bi0.5TiO3. The measurements were performed on as sintered and heat treated in vacuum samples. The diffuse dielectric anomalies associated with the structural phase transitions were observed in as sintered samples. The intense peak of permittivity (εmax ∼ 104) appeared after heat treating in vacuum. The anomaly of ε(T) was contributed by slow polarization processes (f < 10 kHz) and was non-stable, vanishing on heating in air up to ∼ 800 K. Temperature and frequency dependencies of ε were described by using Cole-Cole model with accounting thermally stimulated decay of the non-stable polarization. It is supposed that the dielectric anomaly is determined by space charge polarization mechanism. Oxygen vacancies VO•• and electrons localized on titanium ions Ti'Ti are assumed to be responsible for the phenomenon observed.

Within unitary transformed Hamiltonian of Fröhlich type, using the Green's functions method, exact renormalized energy spectrum of quasiparticle strongly interacting with two-mode polarization phonons is obtained at T=0 K in a model of the system with limited number of its initial states. Exact analytical expressions for the average number of phonons in ground state and in all satellite states of the system are presented. Their dependences on a magnitude of interaction between quasiparticle and both phonon modes are analyzed.