Name
OCI — Expand space of single excitations in a CI calculation.
Description
OCI=n
OCI is only active in the context of some CI job (explicit or implicit) with an active space
of MO orbitals (a through b). In addition to
the determinants generated in the primary CI active space, single excitations will be added at start from the
n MOs just below the limit a, and to the
n MO just higher than the limit b. For example, a
SDTC.I.(a,b) will
have single, double, and triple excitations on orbitals a through b and including OCI=n would
add single excitations for orbitals from a - n to b + n.
With respect to the hard core of a CI in a weak active space, OCI may offer a larger active space in which a
first order repolarization of MOs is allowed. In some sense, it is an “Onion CI”.
OCI(n,m)
Same as OCI=n, except that n orbtials before
and m orbitals after the active space are used (i.e. orbitals from
a - n to b + m).
