Isotope effect on hydrogen bond symmetrization in hydrogen and deuterium fluoride crystals by molecular dynamics simulation.

The isotope effect on the collective proton/deuteron transfer in hydrogen and deuterium fluoride crystals has been investigated at 100 K by ab initio quantum-thermal-bath path-integral molecular dynamics (QTB-PIMD) simulation. The deuterons within a planar zigzag chain of the orthorhombic structure simultaneously flip between covalent and hydrogen bonds due to the barrier crossing through tunnelling. The height of the corresponding static barrier normalized for one deuteron is 29.2 meV. In the HF crystal, all the protons are located at the center of the heavy-atom distance. This evidences the symmetrization of the H-bonds, and indicates that the proton zero-point energy is above the barrier top. The decrease of the heavy-atom distance due to quantum fluctuations in both HF and DF crystals corresponds to a large decrease and an increase of the hydrogen and covalent bond lengths, respectively. Upon deuteration, the increase of the heavy-atom distance (Ubbelohde effect) is in agreement with experimental data.

Zero-Point Energy Leakage in Quantum Thermal Bath Molecular Dynamics Simulations.

The quantum thermal bath (QTB) has been presented as an alternative to path-integral-based methods to introduce nuclear quantum effects in molecular dynamics simulations. The method has proved to be efficient, yielding accurate results for various systems. However, the QTB method is prone to zero-point energy leakage (ZPEL) in highly anharmonic systems. This is a well-known problem in methods based on classical trajectories where part of the energy of the high-frequency modes is transferred to the low-frequency modes leading to a wrong energy distribution. In some cases, the ZPEL can have dramatic consequences on the properties of the system. Thus, we investigate the ZPEL by testing the QTB method on selected systems with increasing complexity in order to study the conditions and the parameters that influence the leakage. We also analyze the consequences of the ZPEL on the structural and vibrational properties of the system. We find that the leakage is particularly dependent on the damping coefficient and that increasing its value can reduce and, in some cases, completely remove the ZPEL. When using sufficiently high values for the damping coefficient, the expected energy distribution among the vibrational modes is ensured. In this case, the QTB method gives very encouraging results. In particular, the structural properties are well-reproduced. The dynamical properties should be regarded with caution although valuable information can still be extracted from the vibrational spectrum, even for large values of the damping term.

Quantum Thermal Bath for Path Integral Molecular Dynamics Simulation.

The quantum thermal bath (QTB) method has been recently developed to account for the quantum nature of the nuclei by using standard molecular dynamics (MD) simulation. QTB-MD is an efficient but approximate method when dealing with strongly anharmonic systems, while path integral molecular dynamics (PIMD) gives exact results but in a huge amount of computation time. The QTB and PIMD methods have been combined in order to improve the PIMD convergence or correct the failures of the QTB-MD technique. Therefore, a new power spectral density of the random force within the QTB has been developed. A modified centroid-virial estimator of the kinetic energy, especially adapted to QTB-PIMD, has also been proposed. The method is applied to selected systems: a one-dimensional double-well system, a ferroelectric phase transition, and the position distribution of an hydrogen atom in a fuel cell material. The advantage of the QTB-PIMD method is its ability to give exact results with a more reasonable computation time for strongly anharmonic systems.

Isotope effects in lithium hydride and lithium deuteride crystals by molecular dynamics simulations.

Molecular dynamics (MD) simulations have been carried out to study isotope effects in lithium hydride and lithium deuteride crystals. Quantum effects on nuclear motion have been included through a quantum thermal bath (QTB). The interatomic forces were described either within the density functional theory (DFT) in the generalized gradient approximation (GGA) or by the phenomenological approach using the shell model. For both models, the isotopic shift in the lattice parameter can be successfully predicted by QTB-MD simulations. The slope of the experimental isotopic shift in pressure is satisfactorily reproduced by QTB-MD within DFT-GGA, in contrast to both density functional perturbation theory and QTB-MD with the shell model. We have analyzed the reasons for these discrepancies through the vibrational densities of states and the isotopic shifts in bulk modulus. The results illustrate the importance of anharmonic contributions to vibrations and to the isotopic pressure shift between LiH and LiD.

Saturation of the ion-hammering effect for large non-hydrostatic capillarity stresses in colloidal silica nanoparticles.

We investigate the role of capillarity stresses on the ion-hammering phenomenon when sub-micrometer colloidal particles are considered. To this end, nearly monodisperse, chemically synthesized silica (SiO2) colloids (100, 300 and 600 nm) were irradiated at room temperature (300 K) with 4 MeV Au ions for fluences up to Φ = 1.8 × 10¹6 cm⁻². It has been taken for granted that the transverse dimension of an ion-deformable amorphous material grows exponentially with the irradiation fluence, L(φ) = L0exp[A0Φ]. Here, we show that for sub-micrometer particles the irradiation-induced deformation saturates for larger fluences, L(φ)→const. The saturation fluence depends on the initial dimension of the colloidal nanoparticles: the smaller the dimension of the colloids, the lower the saturation fluence. Experimental data are successfully accounted for by having recourse to a phenomenological model first developed by Klaumünzer and further elaborated by van Dillen. We also estimate the evolution with fluence of the principal stresses inside the particles, σ11(φ) = σ22(φ) and σ33(φ), and we show that they evolve toward a steady-state value following a sigmoidal-like behavior. Furthermore, when stresses induced by the surface curvature become non-negligible the approximation often made that the deformation strain rate, A0 = dL/L dΦ, remains constant upon irradiation is no longer valid. We show that A0 evolves with the irradiation fluence, e.g. A0→A(Φ), and we relate this behavior to the evolution of the stresses upon irradiation. Finally, this work allows us to define the limits of the ion-hammering effect when the non-hydrostatic capillarity stresses become important.

Thermodynamics and kinetics of the Schottky defect at terraces and steps on the MgO(001) surface.

The Schottky defects at both the flat MgO(001) surface and the monatomic-step edge have been investigated by equilibrium molecular dynamics simulations. The formation enthalpy as a function of the distance between the Mg and O vacancies that form a Schottky defect have been calculated and discussed. We conclude that a step edge is a stable location for a vacancy. The migration mechanism has been elucidated and an intermediate state has been identified. The associated activation enthalpies have been determined in the 700-1100 K temperature range. Both magnesium and oxygen vacancies at the surface are very mobile and can play a role during the crystal growth.

Competing mechanisms in the atomic diffusion of a MgO admolecule on the MgO(001) surface.

The diffusion mechanism of a MgO admolecule on a flat MgO(001) surface has been investigated by equilibrium molecular dynamics simulation. Care has been taken in the choice of the phenomenological interionic potential used. Four distinct mechanisms have been found and the corresponding dynamical barriers determined at high temperature. Some static barriers have also been computed for comparison and all intermediate configurations have been obtained with the same phenomenological potential and also by the DFT-GGA approach. The hopping mechanisms involving the Mg adatom, although dominant, must be combined with the infrequent mechanisms involving displacements of O adatoms in order to provide the mass transport on the surface, which is crucial for crystal growth both in the nucleation and step-flow regimes.

Quantum thermal bath for molecular dynamics simulation.

Molecular dynamics (MD) is a numerical simulation technique based on classical mechanics. It has been taken for granted that its use is limited to a large temperature regime where classical statistics is valid. To overcome this limitation, the authors introduce in a universal way a quantum thermal bath that accounts for quantum statistics while using standard MD. The efficiency of the new technique is illustrated by reproducing several experimental data at low temperatures in a regime where quantum statistical effects cannot be neglected.

Effects of localization of the semicore density on the physical properties of transition and noble metals.

We use density functional theory to study the density of the 3sp semicore states in transition and noble metals. The first objective is to understand how semicore states influence cohesive properties which mainly depend on the valence density. We define a localization radius for the semicore density which is found to be a crucial parameter and to heavily influence the cohesive properties. The localization radius is found to be controlled by the occupation numbers of the 3d states. This offers the possibility of setting criteria for the construction of accurate large core pseudopotentials freezing the semicore states, and to a posteriori control the use of large core pseudopotentials in the modeling. We illustrate our findings with the examples of copper and titanium.