which mosfet diode model should i use?

[henrywent]

I use a CMOS 0.18um process library to perform my hspice simulation. From the hspice user's manual i saw four different mosfet diode models for simulation, i.e.
ACM=0,ACM=1,ACM=2 and ACM=3, which model should i use in my design and how to specify it ? plz help!