The IBM XL C/C++ compiler provides a significant performance boost to NAMD on PowerPC G4 and G5 processors. Unfortunately, the runtime library cannot be statically linked, so you must download it from http://ftp.software.ibm.com/aix/products/ccpp/vacpp-rte-macos/ and install it based on the the README file in that directory. If the library is not installed, running NAMD will product this error:
dyld: ./namd2 can't open library: /opt/ibmcmp/lib/libibmc++.A.dylib