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 ?





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> 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 & DFT up to 16 physical cores;
* scale well for MP2 up to 8 physical cores;
* almost do not scale for CI & 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](http://biowulf.nih.gov/apps/gaussian/#memory)).


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