A couple additional notes--
* In general, constant P methods often require the use of PBC to impose a volume that can change in size in response to the pressure.
* CPT is also a CHARMM keyword for a particular algorithmic approach (extended system) that can be used to perform simulations in a variety of statistical mechanics ensembles, including NPT, NVT, NPH, and specialized anisotropic ensembles such as NPAT or NPgT for lipid bilayers and other systems with a planar interface.
* The CPT keyword requires the use of CRYSTAL for PBC
* I suspect no one knows exactly what you mean by "optimize a protein"; I know I don't.