![]() ![]() In practice it turns out that adiabaticity can be maintained only if the ionic and electronic subsystems are effectively decoupled because the existence of an energy-gap at the Fermi-level leads to largely different time-scales for electronic and ionic excitations. ![]() However, especially for metals, some serious difficulties remain: (a) The original CP approach is based on a coupled pseudo-Newtonian dynamics for the ionic coordinates and for the electronic wavefunctions and requires a careful control of the adiabaticity of the system. The work of Car and Parrinello has triggered many fruitful developments and today we are able to treat systems and processes on the basis of fully quantum-mechanical simulations that are far more complex than has been thought to be possible before. A remarkable step on this way was realized eleven years ago when Car and Parrinello (CP) pointed out that the ionic and electronic degrees of freedom of a system need not to be treated separately, but may be optimized simultaneously. For more than thirty years the local-density-approximation (LDA) has formed the basis of the progress towards an ab-initio description of complex processes in materials.
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