one_rdm_t Derived Type

    type one_rdm_t
        ! The 1-RDM object itself.
        real(dp), allocatable :: matrix(:, :)

        ! Eigenvalues of the 1-RDM.
        real(dp), allocatable :: evalues(:)
        ! Arrays to hold the diagonal of the 1-RDM, and the Lagrangian.
        real(dp), allocatable :: rho_ii(:)
        real(dp), allocatable :: lagrangian(:, :)

        integer :: rho_ii_tag, matrix_tag, evalues_tag

        ! In the 1-RDM matrix array, elements are not stored in the same order
        ! as the orbitals in the rest of the code. Instead, orbitals are
        ! reordered so that orbitals are first sorted by their symmetry label.
        ! sym_list_no(i) holds the position of the corresponding row/column
        ! in the RDM for orbital i. i.e. orbital i is mapped to sym_list_no(i).
        integer, allocatable :: sym_list_no(:)
        ! The inverse of sym_list_no, i.e. what orbital does row or column i
        ! in the 1-RDM correspond to?
        integer, allocatable :: sym_list_inv_no(:)
    end type one_rdm_t