The physical significance of imaginary phonon modes in crystals

Ioanna Pallikara, Prakriti Kayastha, Jonathan M Skelton*, Lucy D Whalley*

*Corresponding author for this work

Research output: Contribution to journalReview articlepeer-review

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Abstract: The lattice vibrations (phonon modes) of crystals underpin a large number of material properties. The harmonic phonon spectrum of a solid is the simplest description of its structural dynamics and can be straightforwardly derived from the Hellman–Feynman forces obtained in a ground-state electronic structure calculation. The presence of imaginary harmonic modes in the spectrum indicates that a structure is not a local minimum on the structural potential-energy surface and is instead a saddle point or a hilltop, for example. This can in turn yield important insight into the fundamental nature and physical properties of a material. In this review article, we discuss the physical significance of imaginary harmonic modes and distinguish between cases where imaginary modes are indicative of such phenomena, and those where they reflect technical problems in the calculations. We outline basic approaches for exploring and renormalising imaginary modes, and demonstrate their utility through a set of three case studies in the materials sciences.
Original languageEnglish
Article number033002
Number of pages19
JournalElectronic Structure
Issue number3
Early online date20 Jul 2022
Publication statusPublished - 1 Sept 2022


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