Jump-Precursor State and a Lengthening Rate-Exchange Time in the Crossover Region of Supercooled <i>o</i>-Terphenyl

There is a “crossover” in the nature of relaxation in the middle of the supercooled liquid range. We have re-analyzed millisecond-long simulations of o-terphenyl [J. Phys. Chem. B 117, 12898 (2013)] with new statistical methods that provide more molecular detail about this crossover. These simulations reach α-relaxation times τ_α that are two orders-of-magnitude slower than at the mode-coupling temperature T_MCT. The rotational Green’s function shows that as the temperature drops below T_MCT, the rotational-jump size lengthen, and an unexpected jump-precursor state emerges. The rate-exchange time τ_ex is measured using multidimensional correlation functions. It is below the α-relaxation time, τ_ex < τ_α , at T_MCT, but slows to τ_ex = τ_α at the lowest temperature simulated. The same methods applied to single-molecule data [Phys. Rev. E 98, 040603(R) (2018)] show that the slowing continues to τ_ex = 22 τ_α close to the glass transition. Overall, a broad crossover is seen, which involves multiple phenomena, including ones unique to the crossover region.