I have a quite big set of molecules to optimize and study and, after some research in the literature, I selected B3LYP/6-311G* level of theory.
These molecules are quite big and wobbly, so I expected the optimization to be rather difficult. However, for most of them, I achieved the optimization with tight threshold (# opt=verytight b3lyp/6-311g* int=ultrafine). However, for some of them, the calculation does not converge. In particular, the SCFs converge with small number of steps but the optimization seems to never reach a minimum.
For this reason, I tried different things: start from a slightly different configuration to 'escape' from that point in the PES, a slightly larger basis set, to change a little the threshold with IOP keyword, the Opt=CalcFC option. None worked. The only way I could obtain a minimum was to lift the accuracy and use the default threshold for the optimization (# opt b3lyp/6-311g*).
I plan to calculate the UV-Vis of these molecules. My question is: Is there another thing that I could try to optimize these systems and/or the default opt parameters would be just fine in general cases?