shoot/rk4.f at main · sscollis/shoot · GitHub

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C***********************************************************************
C23456789012345678901234567890123456789012345678901234567890123456789012
C***********************************************************************
      subroutine RK4(neq, yo, yf, to, h, FUNC)
C***********************************************************************
C
C     Advance one time step using fourth-order (complex) Runge-Kutta
C
c***********************************************************************
      external FUNC
      integer  neq
      real     to, h
      complex  yo(neq), yf(neq)
      complex  f(neq), k1(neq), k2(neq), k3(neq), k4(neq), q(neq)

      call FUNC(neq, yo, to, f)
      do j = 1 , neq
        k1(j) = h*f(j)
        q(j) = yo(j) + 0.5*k1(j)
      end do
      call FUNC(neq, q, to+0.5*h, f)
      do j = 1 , neq
        k2(j) = h*f(j)
        q(j) = yo(j) + 0.5*k2(j)
      end do
      call FUNC(neq, q, to+0.5*h, f)
      do j = 1 , neq
        k3(j) = h*f(j)
        q(j) = yo(j) + k3(j)
      end do
      call FUNC(neq, q, to+h, f)
      do j = 1 , neq
        k4(j) = h*f(j)
        yf(j) = yo(j)+k1(j)/6.+(k2(j)+k3(j))/3.+k4(j)/6.
      end do

      return
      end