GAMESS and Gaussian : what is the maximum efficient number of supported nodes? I have looked at GAMESS and Gaussian manuals but can not find the maximum number of efficient nodes. As I tried GAMESS on our supercomputer, I can add as many CPUs as I want but it really does not Improve the calculation time and efficacy . On the other hand just adding maximum number of CPUs is just waste of CPU-Hour time and resources. 

Is there any criteria for selection of best number of CPUs for optimum resource usage ?
Is there any criteria for selection of best number of CPUs for optimum
  resource usage?


The criteria is basically the same as for choosing the level of theory: benchmarking. Parallel scaling depends on many factors: the level of theory, the size of the molecule, the machine architecture, etc., so there is no golden rule.

Though from my experience I can say that Gaussian 09 single point energy, geometry optimisation and frequencies calculations:


scale well for HF &amp; DFT up to 16 physical cores;
scale well for MP2 up to 8 physical cores;
almost do not scale for CI &amp; CC.


Also note that Gaussian 09 is about 2 times faster for almost any kind of calculations than the previous version (Gaussian 03) and doesn't suffer from some awkwardly strange memory effects (see e.g. here).