tstep.f

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      REAL*8 FUNCTION tstep(F,FDOT,F2DOT,F3DOT,ETA)
*
*
*       General time-step criterion.
*       ----------------------------
*
      IMPLICIT REAL*8  (a-h,o-z)
      REAL*8  f(3),fdot(3),f2dot(3),f3dot(3)
*
*       Obtain new integration step using composite expression.
*       STEP = (ETA*(F*F2DOT + FDOT**2)/(FDOT*F3DOT + F2DOT**2))**0.5.
*
      f2 = f(1)**2 + f(2)**2 + f(3)**2
      fdot2 = fdot(1)**2 + fdot(2)**2 + fdot(3)**2
      f2dot2 = f2dot(1)**2 + f2dot(2)**2 + f2dot(3)**2
      f3dot2 = f3dot(1)**2 + f3dot(2)**2 + f3dot(3)**2
*
      tstep = (sqrt(f2*f2dot2) + fdot2)/(sqrt(fdot2*f3dot2) + f2dot2)
      tstep = sqrt(eta*tstep)
*
      RETURN
*
      END