Modeling disorder in solids
Many crystal structures possess static positional disorder. Materials Studio allows you to model disorder by defining mixture atoms.
Mixture atoms
Atomic sites in a disordered crystal can be described in terms of a hybrid atom, which consists of two or more element types. The relative concentrations can be set for any number of atoms, where the total concentration must be 100%. The mixture atoms description is the most often used representation of solid solutions, metallic alloys, disordered minerals, and so on.
Technical limitations
Current implementation of the molecular dynamics (MD) algorithms in CASTEP is not yet compatible with the virtual crystal approximation (VCA) formalism, which is used to model disorder in solids. None of the MD schemes (NVE, NPT, and so on) work for disordered crystals in the current version.
The following properties are unavailable for disordered systems:
- Population analysis
- Vibrational properties (phonon dispersion, phonon density of states)
- Optical properties
In addition, geometry optimization based on damped MD is not available (since MD itself is not allowed).
Atom occupancy cannot be used with the current CASTEP implementation of the virtual crystal approximation (VCA). Partial atom occupancy can be considered as a special case of a mixture atom, where one part of the mixture is an atom of "nothing". This case is not supported for a number of technical and fundamental reasons.
The limitations listed above fall into three categories:
- Properties that are ill-defined or do not make sense at all for mixture atoms (population analysis, vibrational properties).
- Features where the calculations might be possible, but which require more theoretical work (optics, partial atomic occupancies).
- Features that are blocked for purely technical reasons (molecular dynamics).
These limitations are imposed by the user interface; if you attempt a run using a combination of settings which is not allowed, a message box explaining why the run cannot be carried out will be displayed when you select Run or Save Files.