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Studies of the liquid state present an obvious fundamental interest and are also important for technological applications since the molten state is an essential stage in various industrial processes (e.g. glass making, single crystal growing, iron and steel making). Most of the physical properties of a high-temperature liquid are related to its atomic structure. Thus it is important to develop devices to probe the local environment of the atoms in the sample. At very high temperature, it is difficult to use conventional furnaces, which present several problems. In particular, physical contact with the container can contaminate the sample and/or modify its structural properties. Such problems encouraged the development of containerless techniques, which are powerful tools to study high-temperature melts. By eliminating completely any contact between sample and container, it is possible to study the sample with a very high degree of control and to access very high temperatures. An additional advantage of levitation methods is that it is possible to supercool hot liquids down to several hundred of degrees below their equilibrium freezing point, since heterogeneous nucleation processes are suppressed.

The breathing of the flexible metal organic framework MIL-53(Cr) has been widely explored by both experimental and modeling approaches upon the inclusion of different guest molecules within its porosity. This spectacular phenomenon has been only partially tackled by force field based simulations mainly due to the complexity of deriving a set of accurate potential parameters able to capture the associated structural transition implying a unit cell volume change up to 40%. Here, a new parametrization of a flexible force field for the MIL-53(Cr) framework is realized from an iterative procedure starting with the experimental structural data collected in the presence of CO 2 as the guest molecule. Hybrid osmotic Monte Carlo simulations based on this refined force field are then successfully conducted to reproduce for the first time the complex shape of the CO 2 adsorption isotherm in the whole range of pressures. The structural behavior of the MIL-53(Cr) under a wide range of applied temperature and pressure is then followed by molecular dynamics simulations. It is established that these two stimuli also induce a similar reversible structural transition toward a contracted phase, with the presence of a hysteresis. Each of these predictions is confirmed by experimental evidence issued from either the literature for the impact of the temperature or from our own high pressure neutron diffraction measurements. This permanent experimental/modeling interplay allows a full validation of the derived flexible force field, a prerequisite for further understanding the microscopic key features that govern the spectacular breathing of such a material. © 2012 American Chemical Society.