19 research outputs found
Simulation of Material Properties Below the Debye Temperature: A Path-Integral Molecular Dynamics Case Study of Quartz
Classical and path integral molecular dynamics (PIMD) simulations are used to
study alpha-quartz and beta-quartz in a large range of temperatures at zero
external stress. PIMD account for quantum fluctuations of atomic vibrations,
which can modify material properties at temperatures below the Debye
temperature. The difference between classical and quantum mechanical results
for bond lengths, bond angles, elastic modulii, and some dynamical properties
is calculated and comparison to experimental data is done. Only quantum
mechanical simulations are able to reflect the correct thermomechanical
properties below room temperature. It is discussed in how far classical and
PIMD simulations can be helpful in constructing improved potential energy
surfaces for silica.Comment: 8 pages, 9 figures, submitted to J. Chem. Phy
Interatomic potentials: Achievements and challenges
Interactions between atoms can be formally expanded into two-body,
three-body, and higher-order contributions. Unfortunately, this expansion is
slowly converging for most systems of practical interest making it inexpedient
for molecular simulations. This is why effective descriptions are needed for
the accurate simulation of many-atom systems. This article reviews potentials
designed towards this end with a focus on empirical interatomic potentials not
necessitating a-priori knowledge of what pairs of atoms are bonded to each
other, i.e., on potentials meant to describe defects and chemical reactions
from bond breaking and formation to redox reactions. The classes of discussed
potentials include popular two-body potentials, embedded-atom models for
metals, bond-order potentials for covalently bonded systems, polarizable
potentials including charge-transfer approaches for ionic systems and
quantum-Drude oscillator models mimicking higher-order and many-body
dispersion. Particular emphasis is laid on the question what constraints on
materials properties ensue from the functional form of a potential, e.g., in
what way Cauchy relations for elastic tensor elements can be violated and what
this entails for the ratio of defect and cohesive energies. The review is meant
to be pedagogical rather than encyclopedic. This is why we highlight potentials
with functional forms that are sufficiently simple to remain amenable to
analytical treatments, whereby qualitative questions can be answered, such as,
why the ratio of boiling to melting temperature tends to be large for
potentials describing metals but small for pair potentials. However, we abstain
for the most part from discussing specific parametrizations. Our main aim is to
provide a stimulus for how existing approaches can be advanced or meaningfully
combined to extent the scope of simulations based on empirical potentials
Static and Dry Friction due to Multiscale Surface Roughness
It is shown on the basis of scaling arguments that a disordered interface
between two elastic solids will quite generally exhibit static and "dry
friction" (i.e., kinetic friction which does not vanish as the sliding velocity
approaches zero), because of Tomlinson model instabilities that occur for small
length scale asperities. This provides a possible explanation for why static
and "dry" friction are virtually always observed, and superlubricity almost
never occurs