Investigating finite-size effects in molecular dynamics simulations of ion diffusion, heat transport, and thermal motion in superionic materials

TL;DR

This study examines how the size of the simulation box affects ion diffusion, heat transport, and thermal motion in superionic materials, revealing significant finite-size effects in certain conductors and minimal effects in others.

Contribution

It provides a systematic analysis of finite-size effects in molecular dynamics simulations of superionic conductors, highlighting the importance of system size for accurate property prediction.

Findings

Diffusivity depends strongly on system size in type-II superionic conductors.

Superionic transition temperature shifts with system size in small cells.

Finite-size effects on thermal conductivity are significant, especially in smaller cells.

Abstract

The effects of the finite size of the simulation box in equilibrium molecular dynamics simulations are investigated for prototypical superionic conductors of different types, namely the fluorite-structure materials PbF2, CaF2, and UO2 (type-II), and the {\alpha} phase of AgI (type I). Largely validated empirical force-fields are employed to run ns-long simulations and extract general trends for several properties, at increasing size and in a wide temperature range. This work shows that, for the considered type-II superionic conductors, the diffusivity dramatically depends on the system size and that the superionic regime is shifted to larger temperatures in smaller cells. Furthermore, only simulations of several hundred atoms are able to capture the experimentally-observed, characteristic change in the activation energy of the diffusion process, occurring at the order-disorder transition to the superionic regime. Finite-size effects on ion diffusion are instead much weaker in {\alpha}-AgI. The thermal conductivity is found generally smaller for smaller cells, where the temperature-independent (Allen-Feldman) regime is also reached at significantly lower temperatures. The finite-size effects on the thermal motion of the non-mobile ions composing the solid matrix follow the simple law which holds for solids.