We present an experimental and simulation study of the collective dynamics of liquid ${\mathrm{D}}_2$ in the range of exchanged wave vectors $3\le Q\le 14{\mathrm{nm}}^{-1}$. Neutron Brillouin scattering results for the center-of-mass dynamic structure factor of this moderately quantum fluid are compared, on an absolute scale, with those obtained by three different quantum simulation methods such as ring polymer molecular dynamics and two versions of the Feynman-Kleinert approach. The experimental data can be well described by dynamical models typically used for liquids. Some discrepancies show up both among simulations, and between simulations and experimental data. Such discrepancies are found to mainly concern the damping of the propagating sound modes.

Despite the simplicity of water molecules, for more than a century, a lot of scientific effort has been made on exploring ice polymorphism, especially more elusive phases such as cubic ice Ic. In this work, measurement of the density of phonon states (DOPSs) of polycrystalline ice Ic, and of its deuterated counterpart, has been performed in a sample having an almost perfect crystallographic purity, obtained from the transformation of ice XVII, and with a remarkable accuracy. Results are compared with the new accurate measurements of DOPSs in ice Ih and with centroid molecular dynamics (CMD) simulations. The differences between the experimental DOPSs in these two forms of ice are subtle but quantitatively measurable. In addition, they are reproduced semiquantitatively by computational methods, demonstrating the effectiveness of this innovative simulation tool for calculating the dynamical properties of ice structures.

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors $Q$ between the upper boundary of the hydrodynamic region and $Q_{\mathrm{p}}/2$, with $Q_{\mathrm{p}}$ being the position of the main peak of the static structure factor. In the $Q$ range studied, we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition. The decomposition into exponential modes allows one to convincingly establish not only the crossover related to the onset of transverse waves but, surprisingly, also the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher $Q$ values, a similarly smooth over to underdamped transition.

Hydrogen clathrate hydrates are ice-like crystalline substances in which hydrogen molecules are trapped inside polyhedral cages formed by the water molecules. Small cages can host only a single H2 molecule, while each large cage can be occupied by up to four H2 molecules. Here, we present a neutron scattering study on the structure of the sII hydrogen clathrate hydrate and on the low-temperature dynamics of the hydrogen molecules trapped in its large cages, as a function of the gas content in the samples. We observe spectral features at low energy transfer (between 1 and 3 meV), and we show that they can be successfully assigned to the rattling motion of a single hydrogen molecule occupying a large water cage. These inelastic bands remarkably lose their intensity with increasing the hydrogen filling, consistently with the fact that the probability of single occupation (as opposed to multiple occupation) increases as the hydrogen content in the sample gets lower. The spectral intensity of the H2 rattling bands is studied as a function of the momentum transfer for partially emptied samples and compared with three distinct quantum models for a single H2 molecule in a large cage: (i) the exact solution of the Schrödinger equation for a well-assessed semiempirical force field, (ii) a particle trapped in a rigid sphere, and (iii) an isotropic three-dimensional harmonic oscillator. The first model provides good agreement between calculations and experimental data, while the last two only reproduce their qualitative trend. Finally, the radial wavefunctions of the three aforementioned models, as well as their potential surfaces, are presented and discussed.