An efficient approach for extracting anharmonic force constants from atomistic simulations

F. Eriksson
Master′s Thesis (2018)
doi: 20.500.12380/255598
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Phonon theory is an important tool when analyzing solid state systems which is of high importance in modern technology. Central to the theory is the so called force constants (FCs) which determines the thermal behaviour. The super cell method is one way to extract the FCs from the force field of a displaced specimen but they are computationally expensive by requiring a lot of carefully prepared input data. By constructing an interatomic potential which uses the harmonic and higher order FCss as parameters the FCs can be extracted by a simple fitting procedure. Some small number of quasi random input configurations and the resulting force fields can contain enough information to extract all FCs with comparably low amount of computation. The underlying symmetries of the lattice must be used to reduce the number of free parameters in the model. This thesis demonstrates the implementation and application of the Force Constant Model. The model performs well and anharmonic FCs can be extracted. Given the FCs the model can be used as a potential and molecular dynamics can be performed yielding a direct method for computing thermal properties. Possible applications include analysis of thermal stabilization and phonon life time determination.