[mpich-discuss] How does MPICH spawn processes, and is there a way to configure it?

I have recently built a cluster of Raspberry Pi computers. When this was made up of a master node and 2 slave nodes there was no issue calling mpiexec across the cluster. When I add an additional node to the cluster bringing the size to 1 master and 3 slave nodes I get an error of "Host File Verification Failed". I've tested password-less SSH on all nodes and it's working correctly. I've done some googling of the error and set up and the only mention I've gotten to it was on this link here relating to OpenMPI - http://www.open-mpi.org/community/lists/users/2013/11/22940.php Is there a similar setup for MPICH and if so can someone link me to the correct documentation page? Cheers Scott