There is no simple answer to that question in general, as the choice of the basis set depends a lot on what you want to calculate.

In your case, you seem to be pretty much on the right track, though. If your system is as simple as it sounds, 6-31+G(d,p) should give you decent results. I would choose a functional with dispersion correction, though, such as at least B3LYP-D3, as these model non-convalent interactions better than the normal functionals.

Hi,

As indicated by Mr. Janich it is better to use dispersion corrected functionals. i can also suggest M062X.

your basis already includes diffuse functions for heavy atoms. But your molecules are in interaction, not chemically bonded. So you need to include BSSE correction after optimization.

BSSE : basis set superposition error

good luck.

Simon Janich

Thank you for your answer.

My final task is that getting the Transition State by qst3. After all, two compounds in one system is needed to combine and then I optimize this by same basis set by which I optimize separated two compounds. The B3LYP-D3 can be used with combined compound?

In my system, the main Lithium complex has 41 atoms without one Lithium which is combined with two carbonyl oxygens. Without Lithium the main compound is organic compound. The total reaction is acid-base reaction.

Berkay Sütay

Thank you for your answer.

My final task is that getting the Transition State by qst3 and compare each energy.

So, need to use the same basis set with each calculation such as separated two molecule system that each molecule has charge and total charge of system is zero, and separated two molecule system that each molecule has no charge and total charge of system is zero.The reaction is acid-base reaction.

Can M062X be used to calculate the separated neutral two molecule too?

Hi,

first of all, you have to perform all the calculations in the same basis set.

secondly, yes you may calculate both individual molecules and your interacting systems by M062X functional.

in addition to the basis set, you have to use the same functional in all computations.

QST2 or QST3 gives you an idea of the structure of the activated complex but after you have find it, you should optimize its structure with your functional and basis set too. Finally BSSE should be calculated.