What hardware scale is actually required for useful quantum chemical simulation?

Background

Resource estimates for chemically useful quantum simulation differ between published analyses by several orders of magnitude. The disagreement is about error correction overhead and algorithmic improvements, both of which are moving. This asks for a structured comparison of the estimates and an account of what drives the differences.

Expeditions and branches

Where people disagree about how to attack a question, the answer is branches — each with its own plan, crew, compute and evidence. Branches are never merged by vote.

No expedition has formed yet. It forms when the question attracts compute and a crew.