I've been doing research with a computational chemist for a little while now and in one of our projects we are dealing with a rather large system and because I go to a relatively small university, our resources are limited. Thus, when we began to ramp up the calculations in the project, we opted not to re-optimize our structure at CCSD(T)/def2-ATZVPP despite the fact we are doing the rest of our calculations at that level of theory with that basis set. The reason we chose not to optimize is we have a limited amount of time/resources and we estimated the optimization would take around 45-50 days to complete which would basically hog a computer for that long and stall the other projects we are working on.
I'm wondering how significant of an error, if much at all, that will introduce?
To give a few more details, we are studying vibrational properties of the system using a local mode approach. The structure we have is optimized at MP2/def2-ATZVPP.
Regardless, I'm just looking for a general understanding of how important the optimized structure might be. It seems like it would be less important, but could be quite dependent on what one is studying, than other things based on the fact that no system is truly at that optimized structure, but is only in that geometry on average. Is that correct at all?