Those errors are related to the usage of the "restart" option in the opt route. Restart "copy" all the old opt parameters from the chk file, the error is due to a mismatch between the old route and the new one.

To solve those errors just remove the restart option and set geom=check to read the geometry from the chk file but not the opt options.

Also, the optimization in cartesian coordinates it is deprecable as it is really slow compared to general internal coordinates (GIC), then the calcfc keyword slows down the calculation by not bringing enough benefit to justify it, at least if you don't need force constants of the first point.

Other things, is there a real need to set scf convergence values other than the default ones? Why are you not using symmetry constraints? You should check if your system belongs to a point group or not, if the answer is yes you must remove nosymm (especially to obtain reliable frequencies) and symmetrize the molecule prior to the optimization with software like gaussview or avogadro. In addition, for a frequency calculation, it is desirable to set "very tight" as the convergence criterion of the optimization in order to be more sure of converging to a minimum so as not to end up with imaginary frequencies after. Finally, 3000 cycles both for the opt and the scf are overkill, if your structure does not converge after 500 steps you should better check your system and probably try a different level of theory.

You colud try

#p b3lyp/6-311++g(d,

p) scf=(xqc, MaxConventionalCycles=60) geom=check opt=(maxcycles=300, verytight) freq

Hi Fabio Biffoli,

Thank you very much for taking the time and efforts to answer my query. I will try to modify my job submission with your suggested keywords. I will update on this more after I have obtained my results.

Unfortunately, I could not really answer why I am not using symmetry constraints as I am really quite new at this and am just working off other suggestions. I will certainly do my best to understand more about this from more expert researchers such as yourself.