Mechanisms underlying the mpemba effect in water from molecular dynamics simulations

Experimentally, quenching from warmer water leads to faster freezing than quenching from colder water-the Mpemba effect. Using molecular dynamics, we find that quenching water from 370 K and above leads to a 100 K density of states (DOS) closer to that of ice than quenching from 300 K and below. Especially we find that the biggest difference is for 80-160 cm^-1 which upon quenching from colder water is much lower than that in ice, while it is much higher than in ice when quenching from warm water. We find that the range of 100-160 cm^-1 corresponds to framework vibrations within a hexamer, suggesting that the water hexamer serves as a nucleus for crystallization. We tested this by fixing one hexamer and quenching slowly from 370 K, leading to increased correlation with pure ice. We also showed that the structure quenched from 370 K evolves to the ice faster than 300 K case. These results suggest that the higher population of water hexamer states in warm water is responsible for the faster crystallization underlying the Mpemba effect.