Timing in seconds for MbCO + 3830 water molecules (14026 atoms), 1000 steps 12-14 A shift on Cray T3E. 

Nodes    time (h)   Speedup   
  
  2         1.48       2
  4         0.73       4.05
  8         0.367      8.065
 16         0.181     16.39
 32         0.095     31.22
 64         0.0537    55.15
128         0.0337    87.91
256         0.025    118.40
This table was made by M. Hodoscek at CEA, Saclay, Paris and with CHARMM c25a3 using EXPAND keyword in pref.dat when compiling and SCALAR OFF command in the input script. The communication was paral2.src routines using PVM send/receive. MPI is 4 times slower! The machine has clock speed of 375 MHz.