Abstract:
Calculation of condensed matter observables including anharmonicity developed rapidly in the past decade. With the development of computation power and methodology, first-principle computation of cubic phonon interactions is now regular, and the quartic interactions have begun to receive more attention. In this study, reliable Hamiltonians up to quartic terms are constructed purely in terms of irreducible derivatives, which are calculated efficiently and precisely using the lone and bundled irreducible derivative approach (LID and BID). In the band insulator system CaF2, the range of validity for self-consistent diagrammatic approaches in the classical limit is assessed using irreducible derivatives molecular dynamics performed with the same Hamiltonian. On the other side, the first-principles computation of peak width is rarely directly compared to inelastic neutron scattering (INS) results and previous comparisons show discrepancies even at room temperature. In this study, it is demonstrated that the finite region in reciprocal space required in INS data analysis must be explicitly accounted for within the calculation in order to draw a meaningful comparison, demonstrating in CaF2, ThO2, and UO2. It is also found that the inclusion of this finite reciprocal space region is important to the proper computation of spectrum using techniques with small sampled regions such as inelastic X-ray scattering (IXS). In the end, we show the computation of effective phonons in systems in which translation symmetry is broken using the unfolding method.
