  We introduce a method for determining the functional form of the stochastic
and dissipative interactions in a dissipative particle dynamics (DPD) model
from projected phase space trajectories. The DPD model is viewed as a coarse
graining of a detailed dynamics that displays a clear time scale separation.
Based on the Mori-Zwanzig projection operator method we derive a consistency
equation for the stochastic interaction in DPD. The consistency equation can be
solved by an iterative boot strapping procedure. Combined with standard
techniques for estimating the conservative interaction, our method makes it
possible to reconstruct all the forces in a coarse grained DPD model. We
demonstrate how the method works by recreating the interactions in a DPD model
from its phase space trajectory. Furthermore, we discuss how our method can be
used in realistic systems with finite time scale separation.
