Note

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PECI

Restricts an active space to single-electron and paired double-electron excitations for configuration interaction calculations (i.e. Paired Electron Configuration Interaction). Paired \(\alpha\) and \(\beta\) electrons are excited from a common doubly-occupied orbital to a common empty orbital.

This active space contains only singlet and triplet states.

Number of configurations for \(n\) doubly-occupied orbitals and \(m\) empty orbitals

excitation type	# of configs
none (RHF ground state)	\(1\)
1 \(\alpha\) electron	\(m n\)
1 \(\beta\) electron	\(m n\)
1 pair of \(\alpha\) & \(\beta\) electrons	\(m n\)