Quantum-mechanical simulations of the high-pressure behaviour of crystals
First-principles theoretical methods, based on the quantum mechanics of electrons in periodic atomic systems, have been an effective tool to predict the ground-state structural and energetic behaviour of crystals for at least two decades. However, more recently the computational implementation of such methods by very efficient computer software and the availability of cheap and powerful hardware have undergone impressive improvements. It is thus now possible to tackle quite complex problems of materials science, earth sciences, environmental and other applied disciplines involving crystalline compounds from a purely theoretical point of view (Pisani, 1996). In this respect, while some quantum-mechanical techniques in principle are able to account also for thermal effects (ab initio molecular dynamics), the most easily available ab initio methods neglect the role of temperature (athermal limit) but can include the physical parameter pressure very straightforwardly. A brief account of such methods will be given here, presenting some examples of applications in the fields of thermodynamics and kinetics of crystalline materials at high pressure. Comparisons of quantum-mechanical simulations with those based on atomistic potentials are available in the literature (Catti et al., 2000).
The interest of ab initio predictions of the structural and elastic properties of minerals and technological materials, and of their stability ranges, at high pressure is clear. Measurement techniques in extreme non-ambient conditions are particularly challenging and often subject to large experimental errors (cf. the Diamond-Anvil-Cell methods). The most severe problems are perhaps faced by calorimetric measurements at high pressure, and also the determination