I am playing around with a prostaglandin molecule (23 heavy atoms) in Gaussian09. My goal is to calculate the vertical ionization potential and electron affinity for the molecule. I already managed to converge the geometry of a neutral molecule (with the invaluable help of @Wildcat and Greg at this forum) using Opt=Tight and Integral(Grid=Ultrafine) as well as Opt(Maxcycle=1000) and SCF(Maxcycle=1000). In the log file I got the long-awaited line "Stationary point found" :) However there are also two warning messages there that make me worry a bit:
1. Warning -- assumption of classical behavior for rotation may cause significant error
2. Warning -- explicit consideration of 37 degrees of freedom as vibrations may cause significant error
Now, the zero-point energy that I am interested in has been reported as:
SCF Done: E(RB3LYP) = -1004.62842050 A.U. after 14 cycles
However, right below the warning messages I found something like:
Zero-point correction= 0.442805 (Hartree/Particle)
Sum of electronic and zero-point Energies= -1004.185615
Now, my specific questions are:
Should I worry too much about these warnings if my resultant Ionization Potentials (IP) and Electron Affinities (EA) look reasonable?
Should I do my IP and EA calculations taking the Zero-point correction (see above) into account and use the value of -1004.185615 as my "true" zero-point energy?
What can I do to eliminate the warning messages mentioned above? I read something about a keyword "freq=HinderedRotor" that could be used in such a case.
I am eager to know how to interpret this whole mess and how to proceed. Thank you for any help you may offer!