linear_elec_ind Function

    function linear_elec_ind(i, j) result(ind)
        integer, intent(in) :: i, j
        integer :: ind

        ! for a general orbital indexing i want to get a linear index
        ! for the electron pairs in the closed shell reference
        ! i could set up a matrix (nbasis, nbasis) for these pairs
        ! or i could make a search in the reference det at which position
        ! (i) and (j) are and use this then to encode the infomation..
        ! because i am lazy i think i will setup the matrix!
        ind = elec_ind_mat(i, j)

    end function linear_elec_ind