# Warning!!: The largest alpha/beta MO coefficient is ...

This is a warning that can typically be ignored for normal DFT, but is relevant for post-HF methods. CCSD is supposed to be more robust but it's good to verify whether it has any effect. It typically means coefficients are too large due to a basis set that is too linearly dependent (possibly due to diffuse functions etc). You can verify with a smaller basis sets that the results don't differ too much if you can. For hardness I found it a bit contradicting since the anions require a large and diffuse basis set to be described, but getting rid of this warning requires shrinking the set. The effects however seemed to be minimal (but calculating strong anions remains a bit of a dodgy thing to do anyways). Source below.

"The warning is most relevant for post-HF calculations like Moller Plesset perturbation theory or coupled cluster, where the accuracy of the result is related to the accuracy of the MO integrals. When you have near linear dependencies in the basis set you will get large MO coefficients related to maintaining orthogonality. The side effect of large coefficients is loss of precision on any existing machine. Depending on the order that you add C1*I1+C2*I2-C3*I3+C4*I4 where C1 and C3 are large and of the same sign, if the integrals are all about the same size and C1*I1-C3*I3 is near zero the contribution from C2 or C4 can be lost if they are added before C3 is subtracted. For HF and DFT calculations the code which checks this is run but seldom is it an issue, it is actually just after the SCF completes. For any post-HF method look to see that the correlation corrections are a moderate fraction of the total energy. It may not be an error but it is worth comparing with a slightly smaller basis or a different correlation method, CCSD is less sensitive than MP4, as a check."[[1]]