Car-Parrinello molecular dynamics simulations of a hydroxyl radical in liquid water have been performed. Structural and dynamical properties of the solvated structure have been studied in details. The partial atom-atom radial distribution functions for the hydrated hydroxyl do not show drastic differences with the radial distribution functions for liquid water. The OH is found to be a more active hydrogen bond donor and acceptor than the water molecule, but the accepted hydrogen bonds are much weaker than for the hydroxide OH- ion. The first solvation shell of the OH is less structured than the water's one and contains a considerable fraction of water molecules that are not hydrogen bonded to the hydroxyl. Part of them are found to come closer to the solvated radical than the hydrogen bonded molecules do. The lifetime of the hydrogen bonds accepted by the hydroxyl is found to be shorter than the hydrogen bond lifetime in water. A hydrogen transfer between a water molecule and the OH radical has been observed, though it is a much rarer event than a proton transfer between water and an OH- ion. The velocity autocorrelation power spectrum of the hydroxyl hydrogen shows the properties both of the OH radical in clusters and of the OH- ion in liquid.

Discrete breathers ͑nonlinear localized modes͒ have been shown to exist in various nonlinear Hamiltonian lattice systems. This paper is devoted to the investigation of a classical d-dimensional ferromagnetic lattice with easy plane anisotropy. Its dynamics is described via the Heisenberg model. Discrete breathers exist in such a model and represent excitations with locally tilted magnetization. They possess energy thresholds and have no analogs in the continuum limit. We are going to review the previous results on such solutions and also to report new results. Among the new results we show the existence of a big variety of these breather solutions, depending on the respective orientation of the tilted spins. Floquet stability analysis has been used to classify the stable solutions depending on their spatial structure, their frequency, and other system parameters, such as exchange interaction and local ͑single-ion͒ anisotropy. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1573611͔The problem of energy localization in spatially distributed systems in condensed matter and biology is an important topic of modern physics. A lot of attention in the last several decades has been devoted to the phenomenon of localization due to spatial disorder. In particular, it is a well-known fact that lattice vibrations can localize themselves on impurities "creating so-called impurity localized modes…. In this paper we deal with the relatively new concept of intrinsic localized modes "discrete breathers…. These objects are spatially localized time-periodic lattice vibrations and their existence in translationally invariant "homogeneous… lattices has been proven rigorously. This remarkable phenomenon occurs in nonlinear lattices "lattices, governed by nonlinear equations of motion… and is based on the fact that the spectrum of the linear waves of the system under investigation is bounded and all possible resonances with the linear spectrum can be avoided. In this paper we are going to report on discrete breathers in classical ferromagnetic lattices with the easy-plane anisotropy. We are going to focus on the new type of solutions which have no continuum "soliton… analogs. Discrete breathers here have interesting spatial structure, consisting of a core of several spins, precessing around the hard axis and of tails, consisting of spins precessing with small amplitudes in the easy plane. These solutions possess energy thresholds so that their energy is separated from the energy of the ferromagnetic ground state by a gap. We also study linear stability of these excitations and how it depends on the spatial structure of the breather.

The aqueous solvation shell of a Na + -Cl − pair is studied using Car-Parrinello molecular dynamics simulations. Water-mediated and contact states of the ion pair are investigated. The first hydration shell of the Na + ion is found to be octahedral with one vacant position for both states. In the contact state one of the water molecules is substituted by the Cl − ion. The first hydration shell of the Cl − is less structured and strongly effected by the proximity of the Na + in the contact state. The oxygen coordination numbers for Na + and Cl − are 4.9 and 5.6 for the water-mediated state, and 3.6 and 6.4 for the contact state. The corresponding hydrogen coordination numbers are 10.9 and 5.2; 9.2 and 3.9, respectively.

Topological defects and dislocations in strongly anisotropic crystals consisting of parallel molecular chains are investigated. Our study is focused on the defects in crystalline polyethelyne, which are formed by transverse displacements of chain molecules ͑mutual substitutions and interlacings of adjacent molecular chains in the polymer crystal͒. It is shown that some of these defects called ''twisted topological solitons'' can propagate with a stationary profile and velocity. To describe the dynamics of these solitons, a model that accounts for the three components of the molecular displacements is suggested. Linear topological defects-dislocations-that incorporate the bending of molecular chains in the crystal are also studied.

A simple planar model for an orientational ordering of threefold molecules on a triangular lattice modelling a close-packed (111) plane of fullerite is considered. The system has 3-sublattice ordered ground state which includes 3 different molecular orientations. There exist 6 kinds of orientational domains, which are related with a permutation or a mirror symmetry. Interdomain walls are found to be rather narrow.The model molecules have two-well orientational potential profiles, which are slightly effected by a presence of a straight domain wall. The reason is a stronger correlation between neighbour molecules in triangular lattice versus previously considered square lattice A considerable reduction (up to one order) of orientational interwell potential barrier is found in the core regions of essentially two-dimentional potential defects, such as a threedomain boundary or a kink in the domain wall. For ultimately uncorrelated nearest neighbours the height of the interwell barrier can be reduced even by a factor of 10 2 .

We consider rotational motion of protons within a hydrogen-bonded zig-zag chain. Each proton is subjected to a Coulomb interaction from the three nearest heavy ions, as well as from the two neighboring protons. The hydrogen bonding is modeled with an additional double-minimum on-site potential. The system admits discrete breather solutions in the gap below the phonon band. The numerically exact procedure using an anticontinuum limit is exploited to obtain these solutions, which appear to be asymmetric due to the asymmetry of the interaction potential. Only single-well orbits are considered. A linear stability analysis is performed. The discrete breather solutions are shown to be linearly stable provided the nonresonance condition is satisfied, and they turn out to be unstable in the region of 2:3 parametric resonance. Phonon-breather solutions are found in the 1:2 resonance region. Two kinds of two-site breather solutions are investigated.

A recent model for proton transfer in hydrogen-bonded chains given by Pang and Müller-Kirsten (Pang X F and Müller-Kirsten H J W 2000 J. Phys.: Condens. Matter 12 885) is critically reconsidered. The model violates a basic symmetry of the system. The meaning of the model parameters is overinterpreted. The model can be applied only to describe the motion of ionic defects. The kink solutions corresponding to bonding defects obtained in this work by Pang and Müller-Kirsten are proven to be incorrect.