Kamaleldeen Nassar

Common options:

1- Try to increase the basis set gradually. first, preoptimize your system at lower level like PM6 semiempirical method... and later using these results as a guess in subsequent computations.

2- Try to increase the maxcycle number for SCF. For example, scf=(maxcycle=512) option.

3- The SCF=QC option is often helpful (slower, but more reliable algorithm)

Melek Hajji, very appropriate answer, thank you very much for your potential answer, also I am benefited from this answer.

Your input structure shows that you use fragments. In this case, generate a guess for wavefunction and use that guess to optimize your structure. Due to these fragments, I think your calculations are long. In the Gaussian manual, example 9.7 on page No 447 is helpful. (Exploring chemistry with electronic structure Methods, 3rd Ed.)

Dear Zaheer

by generating guess, do you mean to carry out the calculation with a low level of theory then use this guess in the DFT? or there is another way?

Use same basis set. see attached procedure