just to let you know:
For 1 processor, 600 Aa and 4 copy per asu, and ~60% solvent, my Ubuntu needed 4.8GB of RAM and additionally 60% of the 1.4 GB Swap.
If I used more than one processor, RAM and Swap was completly filled and execution failed with "Failed memory alloc" after a while.
Unfortunately Ubuntu tends to generate small swap partitions, a solution might be to add a swap file (additionally to the partition...) [http://goo.gl/RSLqT]
Just watch you "system monitor"...