  The exponential scaling of complete active space (CAS) and full configuration
interaction (CI) calculations limits the ability of quantum chemists to
simulate the electronic structures of strongly correlated systems. Herein, we
present corner hierarchically approximated CI (CHACI), an approach to wave
function compression based on corner hierarchical matrices (CH-matrices) -- a
new variant of hierarchical matrices based on a block-wise low-rank
decomposition. By application to dodecacene, a strongly correlated molecule, we
demonstrate that CH matrix compression provides superior compression compared
to a truncated global singular value decomposition. The compression ratio is
shown to improve with increasing active space size. By comparison of several
alternative schemes, we demonstrate that superior compression is achieved by a)
using a blocking approach that emphasizes the upper-left corner of the CI
vector, b) sorting the CI vector prior to compression, and c) optimizing the
rank of each block to maximize information density.
