pub fn calc_o2_matrix_element<'a, 'b, T, SC, F>(
    lowdin_paired_coefficientss: &[LowdinPairedCoefficients<T>],
    o2_opt: Option<&'b ArrayView4<'a, T>>,
    get_jk_opt: Option<&F>,
    structure_constraint: &SC,
) -> Result<T, Error>where
    T: ComplexFloat + ScalarOperand + Product + Display,
    SC: StructureConstraint,
    F: Fn(&Array2<T>) -> Result<(Array2<T>, Array2<T>), Error>,
    'a: 'b,Expand description
Calculates the matrix element of a two-particle operator between two Löwdin-paired determinants.
§Arguments
lowdin_paired_coefficientss - A sequence of pairs of Löwdin-paired coefficients, one for each
subspace determined by the specified structure constraint.
o2_opt - The two-particle operator in the atomic-orbital basis.
structure_constraint - The structure constraint governing the coefficients.
§Returns
The two-particle matrix element.