Relationship between two-level systems and quasilocalized normal modes in glasses

Tunneling two-level systems (TLSs) dominate the physics of glasses at low temperatures. Yet TLSs are extremely rare, and it is thus difficult to directly observe them in silico. Developing simple structural predictors, which can provide markers for determining if a TLS is present in a given glass region, is crucial for a more efficient search. It has been speculated that vibrational quasilocalized modes (QLMs) are closely related to TLSs, and that one can extract information about TLSs from QLMs. In this work we address this possibility. In particular, we investigate the degree to which a linear or nonlinear vibrational mode analysis can predict the location of TLSs independently found by energy landscape exploration. We find that even though there is a notable spatial correlation between QLMs and TLSs, in general TLSs are strongly nonlinear, and their global properties cannot be predicted by a simple vibrational mode analysis.