fix rigid/nve with fix nve

Thanks to both Steve and Axel. Regarding the two integrators, I indeed used them on distinct groups of atoms. I think it's not an issue of relaxation and reorientation (at least not directly) since the external field was turned on during equilibration and the box size is very small in the direction of the field. I suspect perhaps it is related to the fact that the system was equilibrated with individual, coupled heat baths, and then released into an unstable configuration.

Anyway, I will move towards my production runs now, and if I still encounter problems I will post something here. Thanks for the help!