“Know how to solve every problem that has been solved.” “What I cannot create, I do not understand.” — Richard Feynman

QC II · Project 3 — Size consistency: catch CISD failing

Quantum Chemistry

Truncated CI hides a fatal flaw, and the cleanest way to see it is to compute one molecule, then two of them far apart, and check whether the energy simply doubled. FCI passes; CISD fails — and the failure is the entire reason coupled cluster exists.