setup_orb_ind_mat Subroutine

    subroutine setup_orb_ind_mat
        character(*), parameter :: this_routine = "setup_orb_ind_mat"

        integer :: i, j, k, orb_i, orb_j

        if (allocated(orb_ind_mat)) deallocate(orb_ind_mat)

        ASSERT(allocated(mask_virt_ni))

        allocate(orb_ind_mat(nbasis, nbasis))
        orb_ind_mat = 0

        k = 1
        do i = 1, Nvirt
            orb_i = mask_virt_ni(i, 1)
            do j = i + 1, nvirt
                orb_j = mask_virt_ni(j, 1)

                orb_ind_mat(orb_i, orb_j) = k
                k = k + 1
            end do
        end do

        ASSERT(k - 1 == n_virt_pairs)

    end subroutine setup_orb_ind_mat