Background¶
The atoms in a material undergo regular vibrational motion around their equilibrium positions, a phenomenon that is of fundamental importance for the overall behavior of the material. In crystalline solids in particular these vibrations are periodic in nature and can be described using quasi-particles named phonons that represent collective excitations of the crystal lattice.
The most essential ingredient required for analyzing phonons in a material is the set of force constants (FCs). hiPhive enables one to efficiently obtain high order FCs (e.g., of fourth or sixth order) including large and low-symmetry systems. To this end, it employs a supercell approach similar to phonopy [TogTan15], shengBTE [LiCarKat14], or alamode [TadGohTsu14].
hiPhive does not rely on a specific type of input configuration (i.e. enumerated displacements or configurations from MD simulations). Rather it employs advanced optimization techniques that are designed to find sparse solutions, which in the present case reflect the short-range nature of the FCs. If the input configurations are constructed sensibly this approach allows one to obtain FCs using a much smaller number of input configurations and thus to reduce the computational effort, usually in the form of density functional theory (DFT) calculations, considerably. This approach becomes genuinely advantageous already for obtaining second order FCs in large and/or low symmetry systems (defects, interfaces, surfaces, large unit cells etc). hiPhive truly excels when it comes to higher order FCs, for which a strict enumeration scheme quickly leads to an explosion of displacement calculations.
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