ffsim.doubles_excitations_restricted

ffsim.doubles_excitations_restricted(t2)

Restricted doubles excitations operator.

The restricted doubles excitations operator is

\[T_2 = \sum_{ijab} t_{ijab} \left[ \frac12 \left( a^\dagger_{a\alpha} a^\dagger_{b\alpha} a_{j\alpha} a_{i\alpha} + a^\dagger_{a\beta} a^\dagger_{b\beta} a_{j\beta} a_{i\beta} \right) + a^\dagger_{a\alpha} a^\dagger_{b\beta} a_{j\beta} a_{i\alpha}\right]\]

where \(i\) and \(j\) run over occupied orbitals, \(a\) and \(b\) run over virtual orbitals, and \(t_{ijab}\) are the doubles amplitudes.

Parameters:

t2 (ndarray) – The doubles amplitudes tensor of shape (nocc, nocc, nvrt, nvrt), where nocc is the number of occupied orbitals and nvrt is the number of virtual orbitals.

Return type:

FermionOperator

Returns:

The doubles excitations operator.