.. _CdotIdotDdot:

C.I.D.
======

Adjusts the active orbitals for double-electron excitations in an :ref:`INDO` calculation with the :ref:`CISD` keyword specified.

**SYNTAX:** ``C.I.D.=n``

Activate the :math:`\lceil n/2 \rceil` orbitals at or below the chemical potential and the :math:`\lfloor n/2 \rfloor` orbitals above the chemical potential.

**SYNTAX:** ``C.I.D.=(n,m)``

Activate the :math:`n` orbitals closest to the chemical potential containing :math:`m` electron pairs.
This accounting ignores the unpaired electron in systems with an odd number of electrons.

By default, 10 orbitals are activated when :ref:`CISD` is specified (equivalent to ``C.I.D.=10``).

Doubly-excited configurations are generated by adding single-electron excitations to the singly-excited configurations in the active space.
This process requires the active orbitals for double excitations to be a subset of the active orbitals for single excitations,
and the values supplied by ``C.I.D.`` are automatically adjusted to satisfy this constraint.

For a :ref:`CISD` calculation with the active orbitals set by ``C.I.=(p,o)`` and ``C.I.D.=(n,m)``,
the number of doubly-excited configurations is approximately :math:`m(m-n)o(p-o)`.

Even if the :ref:`MAXCI` keyword is used to limit the size of the CI Hamiltonian matrix,
generating and computing the energies of doubly-excited electron configurations can become computationally expensive
if a large number of orbitals are activated by this keyword.