There is some blog entry on creating MPI cluster made of virtual machine nodes http://www.zamaudio.com/?p=837

But I am not sure whether it is an optimal solution from the performance point of view.

http://www.zamaudio.com/?p=837

If you're interested in pure QM, there's really no need to make clusters (unless for the reason of using a queueing system, which helps in managment of jobs or for larger storage). The reason is that QM jobs usually scale pretty bad with the number of processors, and it rarely makes sense to run any jobs with more than 16 cores. Even if I use a cluster for QM calculations I usually limit my self to 8 or 16 cores and make sure that they are on the same node, to make the speed optimal.

There is currently a number of very fast and quite reasonably priced (well...) processors with 6/8/10/12 cores and you can build very officient double-processor machines using them, which should handle most of QM calculations.

Dear Bartosz thank you for your answer. I can use many virtual machines but the problem is the limit of 8 GB for each one. This limit is for all the users of our center Okeanos. So, it is not possible to run jobs which need more memory like a mp2 calculation for a dimer molecule. 

The advantage of using many cpus is the parallel processing.