Correct Quantum Chemistry in a Minimal Basis from Effective Hamiltonians.

We describe how to create ab initio effective Hamiltonians that qualitatively describe correct chemistry even when used with a minimal basis. The Hamiltonians are obtained by folding correlation down from a large parent basis into a small, or minimal, target basis, using the machinery of canonical transformations. We demonstrate the quality of these effective Hamiltonians to correctly capture a wide range of excited states in water, nitrogen, and ethylene and to describe ground and excited state bond breaking in nitrogen and the chromium dimer, all in small or minimal basis sets.