I've been trying to calculate vertical ionization potentials (IP) and electron affinities (EA) for a bunch of prostaglandin derivatives (23-25 heavy atoms). My calculation setup in Gaussian09 looks mostly like that:
# opt=(tight,maxcycle=1000) freq=noraman cphf=noread b3lyp/6-31g(d) geom=connectivity integral=grid=ultrafine scf=maxcycle=1000
As you can guess, I have been struggling with converging the geometry and finding the stationary point for my molecules. Some were kind of stubborn and wouldn't converge, so in desperation I increased the
Maxcycle values for
SCF from a
1000 up to
5000, and I succeeded with some of them. Now, my question is if I need to recalculate all other prostaglandins that have already nicely converged with lower
SCF(Maxcycle) options (I used them in different combinations like
5000)? The thing is that I want to be able to objectivelly compare the IP and EA values between different derivatives.
I am concerned because I remember that someone told me that if I went for
SCF=QC option I would need to recalculate the whole set of molecules with the same keyword option. However, as far as I know
SCF=QC is like a substantial change in the setup as it uses another algorithm for calculations. Hopefully, changing the
SCF(Maxcycle) value is not that dramatic, is it? How about
SCF=Verytight - would it demand recalculating everything or not?
Another thing that bothers me is
Opt=Tight. What if I decided to go for
Opt=Verytight. Would I need then to recalculate all of the molecules with the same setting?
And lastly, what if I changed
Integral(grid=ultrafine, acc2e=12) which is supposedly the most precise approach?