Also, I am wondering does the use of Nosymm affect the results of all gaussian jobs such as optimization, frequency calculation, absorption and fluorescence, ....etc
Hallo, Muhammad!
If your structure has some symmetry elements and gaussian starts geometry optimization within this symmetry group then it should keep it during the whole geometry optimization procedure. Thus if the stable structure of your system has lower symmetry you cannot reach it. In such cases you should use the keyword NoSymm.
Except the optimization mentioned the use of this keyword causes longer computation time because the program cannot use some advantages of molecular symmetry (unnecessary repetition of some integrals computations).
On the other hand the use of the Symmetry keyword does not warrant that your system will keep the original symmetry during the geometry optimization (due to numerical inaccuracy).
Best wishes
mb
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