*******************************************************************
*
*       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
*       Modified 10/16/2006 to allow for driven oscillations
*           new parameters alpha=Fo and omega are now used
*           and are read in by the revised int.for program
*
        SUBROUTINE derivs(time,xin,F)

        double precision time,xin(2),F(2)
        double precision drag, drive  ! drag and drive terms
        double precision vtol
        double precision m, k, g, gamma, alpha, omega
        common /params/ m, k, g, gamma, alpha, omega

        parameter (vtol = 1.0d-10)


*       F(i) are the output derivatives
               F(1) = xin(2)                   ! velocity
                  drag = 2*gamma*xin(2)          ! damping term
                  drive = alpha*dcos(omega*time)/m ! driving force
               F(2) =  - k*xin(1)/m - drag - drive     ! acceleration
500     format ('# frict=0 ',1PE14.6,' > ',1PE14.6,' xin(2)=',1PE14.6)
        return
        end
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