Note
This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.
CIS¶
Restricts the active space to single-electron excitations for configuration interaction calculations (i.e. Configuration Interaction Singles).
The CIS active space is not useful for improving the Hartree-Fock approximation of ground-state properties such as total energies, forces, vibrational frequencies, and relaxed geometries. This phenomenon is explained by Koopmans’ Theorem [10], which shows that the many-electron Hamiltonian does not couple the Hartree-Fock ground state to singly-excited configurations. An active space must include configurations with at least two-electron excitations to improve ground-state properties (e.g. CISD).
excitation type |
# of configs |
|---|---|
none (RHF ground state) |
\(1\) |
1 \(\alpha\) electron |
\(m n\) |
1 \(\beta\) electron |
\(m n\) |