Full showreel

End-to-end render of a representative PyKMC trajectory in Ni–Cr–Al: vacancy diffusion, surface ↔ subsurface exchange, and 1NN in-plane hops, sequenced in the order they occurred during the run.

Event 1 — Surface ↔ subsurface exchange (downward)

Surface atom hops into a subsurface vacancy. Sets the downstream availability of vacancies for the in-plane events.

An atom on the surface layer swaps with the subsurface vacancy directly below it. This event transports a vacancy upward to the surface while moving a surface atom into the bulk — the first ingredient in the chromium-driven enhancement of bulk vacancy concentration.

Event 2 — Surface 1NN in-plane (atom 3975)

First-nearest-neighbour hop in the surface plane. Atom index 3975 — a particular local environment.

A surface atom hops to a first-nearest-neighbour vacancy site within the same surface plane. The local chromium / aluminium configuration around atom 3975 sets this event's barrier and rate; comparing it with the same geometric event at a different atom (Event 3) shows how strongly the barrier varies with local chemistry.

Event 3 — Surface 1NN in-plane (atom 2023)

Same geometric event as Event 2, different atom (idx 2023) — different local chemistry, different barrier.

Atom 2023 makes the same geometric move as atom 3975, but its neighbours differ — so the barrier (and the rate that PyKMC samples it at) differs too. Side-by-side with Event 2, this is the cleanest single-event illustration of the local-environment sensitivity that PyKMC has to resolve at every step.

Event 4 — Surface ↔ subsurface exchange (upward, atom 3234)

Reverse of Event 1: subsurface atom 3234 hops up to the surface. Closes the surface ↔ bulk vacancy cycle.

The subsurface atom at index 3234 hops up into a surface vacancy, swapping the vacancy back into the bulk. Paired with Event 1, this completes the simplest surface ↔ subsurface exchange cycle and is the elementary step that lets vacancies (and segregating Al) shuttle between the surface and the interior.