*******************************************************************
*
*       Derivative Function Vector Calculation   JAJ 10/00
*       DeVries 5.55 and 5.56
*       in general, derivatives could depend on
*       time and all x-vector components
*
        SUBROUTINE derivs(time,xin,F)

        double precision time,xin(2),F(2)
        double precision drag  ! drag term
        double precision vtol
        double precision m, k, g, gamma
        common /params/ m, k, g, gamma

        parameter (vtol = 1.0d-10)

*       xin(1) is the particle position
*       xin(2) is the particle velocity

***********************************************************************
***>    You may change the drag and acceleration equations as         *
***>    necessary or desired using defined constants and the particle *
***>    position and/or velocity (defined above).                     *
***********************************************************************

*       F(i) are the output derivatives
               F(1) = xin(2)                   ! velocity
                  drag = 2*gamma*xin(2)        ! linear velocity drag model
               F(2) =  - k*dsin(xin(1))/m - drag ! acceleration for pendulum
*               F(2) =  - k*xin(1)/m - drag     ! acceleration
500     format ('# frict=0 ',1PE14.6,' > ',1PE14.6,' xin(2)=',1PE14.6)
        return
        end
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