A significant fraction of parallel scientific codes are iterative with barriers between iterations or even between phases of the same iteration. The sender of a message is assured that the receiver is executing exactly the same iteration or phase. This opens up the opportunity to use one-sided communication without synchronization, explicit or implicit, between the sender and receiver of every message. The synchronization inherent in the application is sufficient to ensure correctness. We present CkDirect, an interface for such one-sided communication in the message driven Charm++ runtime system. CkDirect helps avoid unnecessary synchronization and message copying as well as scheduling overhead in iterative scientific codes. We describe the interface as well as its implementations on two different interconnects: Infiniband and Blue Gene/P. We evaluate CkDirect through a micro-benchmark, two simple scientific codes: stencil computation and matrix multiplication, as well as a full fledged quantum chemistry application called OpenAtom.