perturbation Derived Type

    type perturbation
        ! The number of annihilation operators in the perturbation.
        integer :: nannihilate = 0
        ! The orbitals to be annihilated.
        integer, allocatable :: ann_orbs(:)
        ! The elements in the ilut representation where the occupation of the above orbs are encoded.
        integer, allocatable :: ann_elems(:)
        ! The positions of the bits in the bitstring representation where the above orbs are encoded.
        integer, allocatable :: ann_bits(:)

        ! The number of creation operators in the perturbation.
        integer :: ncreate = 0
        ! The orbitals to be created.
        integer, allocatable :: crtn_orbs(:)
        ! The elements in the ilut representation where the occupation of the above orbs are encoded.
        integer, allocatable :: crtn_elems(:)
        ! The positions of the bits in the bitstring representation where the above orbs are encoded.
        integer, allocatable :: crtn_bits(:)
    end type perturbation