!-------------------------------------------------------------------
    ! function  : dcurvepot
    !
    ! package   : potentials
    !
    ! Language  : Fortran 90
    !
    ! author    : F. Colavecchia (flavioc@lanl.gov)
    !
    ! date      : 02/09/03     version:
    ! revision  :              version:
    !
    ! purpose   :  Compute the derivative of the potential curve of arrangement arr
    !              for a internuclear distance r between the 
    !              atoms that compose the arrangement arr.
    !
    ! input     :   arr -> arrangement (1 2 3)
    !               r   -> internuclear distance, NOT mass scalesd
    !
    ! output    :    curvepot -> V_arr(r)/dr in a.u. 
    !
    ! modules   :
    !
    ! common    :
    !
    ! notes     :
    !
    !-------------------------------------------------------------------
    real*8 function dcurvepot(arr,r)
    use pes_module
    !              pesname must contain a character string
    !              of length 20 that properly describes the potential.
    !
    implicit none
    integer arr
    real*8 r,dvnene,vneh
    real*8 rp,delta
    character(len=25) potname

    delta = 1d-3*r
    rp = r+delta
    !
    ! Check for correct potential energy surface specification.
    !
    if(pesname.ne.potname)then
        write(*,*)'The potential energy surface name does not'
        write(*,*)' match the name in the potential energy surface routine.'
        write(*,*)'***error***: Execution stopped in routine curvepot'
        write(*,*) 'pesname=', pesname
        write(*,*) 'potname=', potname
        stop 'curvepot'
    end if
    if(arr.lt.1.or.arr.gt.3) then
        write(*,*) 'Invalid arrangement'
        write(*,*) 'arr =',arr
        stop 'dcurvepot'
    end if
    !
    !
    if(arr.eq.1) then
        dcurvepot = dvnene(r)
    else
        dcurvepot = (vneh(rp)-vneh(r))/delta
    end if
    !  the following is a special case
    !  dcurvepot=0d0
    return
    end