Computational program not using full cpu power
Most applications in theoretical chemistry are memory and or memory bandwidth bound. Especially due to shared memory bandwidth it can be bad for your performance if you use too many processes.

In addition you benefit from additional processes only if your algorithm is more or less parallel. If there are too many serial parts/communication overhead you will have idle cores, if your process number is too high. (Amdahl's law)

Take home message:


Time the execution of your code with a non trivial example.
More is not always better.