ffsim.singles_excitations_unrestricted

ffsim.singles_excitations_unrestricted(t1)[source]

Unrestricted singles excitations operator.

The unrestricted singles excitations operator is

\[T_1 = \sum_{ia} t^{(\alpha)}_{ia} a^\dagger_{a\alpha} a_{i\alpha} + \sum_{IA} t^{(\beta)}_{IA} a^\dagger_{A\beta} a_{I\beta}\]

where

  • \(i\) runs over occupied spin-up orbitals,

  • \(a\) runs over virtual spin-up orbitals,

  • \(I\) runs over occupied spin-down orbitals,

  • \(A\) runs over virtual spin-down orbitals,

  • \(t^{(\alpha})_{ia}\) are the spin-up singles amplitudes, and

  • \(t^{(\beta})_{IA}\) are the spin-down singles amplitudes.

Parameters:

t1 (tuple[ndarray, ndarray]) – The singles amplitudes. This should be a pair of Numpy arrays, (t1a, t1b), containing the spin-up and spin-down singles amplitudes. t1a should have shape (nocc_a, nvrt_a), where nocc_a is the number of occupied spin-up orbitals and nvrt_a is the number of virtual spin-up orbitals. t1b should have shape (nocc_b, nvrt_b), where nocc_b is the number of occupied spin-down orbitals and nvrt_b is the number of virtual spin-down orbitals.

Return type:

FermionOperator

Returns:

The singles excitations operator.