6 research outputs found
Ab initio atomistic thermodynamics and statistical mechanics of surface properties and functions
Previous and present "academic" research aiming at atomic scale understanding
is mainly concerned with the study of individual molecular processes possibly
underlying materials science applications. Appealing properties of an
individual process are then frequently discussed in terms of their direct
importance for the envisioned material function, or reciprocally, the function
of materials is somehow believed to be understandable by essentially one
prominent elementary process only. What is often overlooked in this approach is
that in macroscopic systems of technological relevance typically a large number
of distinct atomic scale processes take place. Which of them are decisive for
observable system properties and functions is then not only determined by the
detailed individual properties of each process alone, but in many, if not most
cases also the interplay of all processes, i.e. how they act together, plays a
crucial role. For a "predictive materials science modeling with microscopic
understanding", a description that treats the statistical interplay of a large
number of microscopically well-described elementary processes must therefore be
applied. Modern electronic structure theory methods such as DFT have become a
standard tool for the accurate description of individual molecular processes.
Here, we discuss the present status of emerging methodologies which attempt to
achieve a (hopefully seamless) match of DFT with concepts from statistical
mechanics or thermodynamics, in order to also address the interplay of the
various molecular processes. The new quality of, and the novel insights that
can be gained by, such techniques is illustrated by how they allow the
description of crystal surfaces in contact with realistic gas-phase
environments.Comment: 24 pages including 17 figures, related publications can be found at
http://www.fhi-berlin.mpg.de/th/paper.htm
Structure and mobility of defects formed from collision cascades in MgO
We study radiation-damage events in MgO on experimental time scales by augmenting molecular
dynamics cascade simulations with temperature accelerated dynamics, molecular statics, and density
functional theory. At 400 eV, vacancies and mono- and di-interstitials form, but often annihilate within
milliseconds. At 2 and 5 keV, larger clusters can form and persist. While vacancies are immobile,
interstitials aggregate into clusters (In) with surprising properties; e.g., an I4 is immobile, but an impinging
I2 can create a metastable I6 that diffuses on the nanosecond time scale but is stable for years
Atomistic simulations of pressure-induced structural transformations in solids
Constant-pressure MD simulations complement constant-volume MD
simulations and naturally allow the study of systems where external
pressure is a driving force for a structural transformation. These
transformations take place in crystalline as well as amorphous systems.
Besides studies of bulk systems there is also growing interest in
simulations of finite systems, such as clusters and nanocrystals, under
pressure. In the paper we review various approaches to constant pressure
simulations with focus on the recent developments in simulation
methodology, such as metadynamics and transition path sampling. The
application of the techniques to bulk and finite systems is illustrated
on several examples