From configuration space to thermodynamic space: Predicting new inorganic solids via global exploration of their energy
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From configuration space to thermodynamic space: Predicting new inorganic solids via global exploration of their energy landscapes J. C. Schön and M. Jansen Max-Planck-Institute for Solid State Research, D-70569 Stuttgart, Germany ABSTRACT We present a general approach to predict new inorganic solid compounds as a function of pressure, composition and temperature. Our method is based on the global exploration of the energy and enthalpy landscapes of the chemical system, followed by the construction of the corresponding free energy/enthalpy landcsapes. INTRODUCTION One of the measures of the depth of understanding achieved in a scientific field is the degree to which one is able to successfully predict the outcome of various experiments based on as little prior information as possible. In the area of solid state chemistry, this translates into the realization of the concept of so-called rational design [1-5] which encompasses the ability to predict the outcome of particular solid state reactions and synthesis routes, the enumeration of (all) (meta)stable modifications that can exist in a chemical system depending on the external thermodynamic conditions, and finally the design of a specific synthesis route towards a predicted compound. While these goals have already been partly realized in synthetic organic chemistry since the nineteen-sixties [6,7], in solid state chemistry we had to wait until the early nineteen-eighties for the formulation of first concepts [1] in this regard and until the early nineteen-nineties for first attempts to put these ideas into practice, first in the context of structure determination for given fixed unit cells [8,9], and then with full global structure prediction [10,11]. Up to now, the focus of rational design has been on the identification of possible synthetic targets for solid state syntheses [3,5,12-14]. All methods employed to predict possible modifications of compounds are based on the idea that such structure candidates correspond to local minima of the energy landscape of the chemical system. The most general approach [3,15,16] consists in the global exploration of these landscapes, which, due to the high computational cost of ab initio energy calculations, is usually split into two steps: a global search of the space of possible configurations using a simplified energy/cost function, followed by a local optimization on ab initio level (or using highly refined empirical potentials). Concurrently with the new theoretical developments, new synthesis methods have been introduced which greatly extend the range of thermodynamic conditions under which compounds can be studied and generated. In particular, synthesis at very high pressures [17,18] and even effective negative pressures [19-22] has become an important area of investigation. As a consequence, one can no longer expect, even for very low temperatures, that all relevant structure candidates at high positive and negative pressures are also local minima of the potential energy, i.e. of the enthalpy at zero pressure.
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