  A new large-scale parallel multiconfigurational self-consistent field (MCSCF)
implementation in the open-source NWChem computational chemistry code is
presented. The generalized active space (GAS) approach is used to partition
large configuration interaction (CI) vectors and generate a sufficient number
of batches that can be distributed to the available nodes. Massively parallel
CI calculations with large active spaces can be treated. The performance of the
new parallel MCSCF implementation is presented for the chromium trimer and for
an active space of 20 electrons in 20 orbitals. Unprecedented CI calculations
with an active space of 22 electrons in 22 orbitals for the pentacene systems
were performed and a single CI iteration calculation with an active space of 24
electrons in 24 orbitals for the chromium tetramer was possible. The chromium
tetramer corresponds to a CI expansion of one trillion SDs (914 058 513 424)
and is largest conventional CI calculation attempted up to date.
