Dear Tian,

Too many thanks for your informative comments.

Sincerely,
Saeed

You must use "genecp" or equivalently, "gen pseudo=read", when ECP is used, otherwise ECP information will not be loaded from the end of the input file.

Dear Tian,
Please suppose we are going to use mixed basis sets for a given compound. For instance, aug-cc-pVTZ for F and aug-cc-pVTZ-PP for I atoms when F-I is going to be optimized. These basis sets can easily be downloaded from "https://www.basissetexchange.org/" and included in a file namely "saeed.gbs". The file "saeed.gbs" would be addressed at the end of Gaussian input file after a blank line.

The very important issue is that when we use mixed basis sets (whether PP basis sets are present or not), employing "genecp" AND NOT "gen" is mandatory in the Gaussian route section. Indeed, the "genecp" keyword guarantees using PP basis sets if any atoms of the systems bear PP bases. If no PP bases are present, the "genecp" keyword would not lead to any problem. As consequence, using "genecp" instead "gen" should be taken into account when mixed basis sets are employed whether or not some atoms of system bear PP bases. Are you agree with this statement?

Sincerely,
Saeed