[vmlxb6]

I have a case in which I am trying to solve a mesh deformation due to fluid forces acting on it. Now there is deflection of the body due to the lift force. The case is very similar to the tutorial example in CFX-12.0 (Ch-22: Fluid structure interaction and mesh deformation)
The tutorial uses lower order EULER's discretisation scheme, but I wanna use RK4th order scheme for the body displacements.
The problem is how do I get the values for t(i-1)+0.5*timestep. I have written a FORTRAN code so I can save values at different timestep but how do I get values at (prev_timestep + 0.5*h).

I hope I am clear. Thank you very much.