You should use the structure you shown as the initial structure to further optimize the dimer and then perform frequency analysis. Directly performing frequency analysis on a point of relaxed scan trajectory is commonly meaningless.

By the way, using B3LYP-D3(BJ) instead of BLYP-D3(0) is a better choice. Moreover, it is highly deprecated to perform relaxed scan or optimization in combination with counterpoise, because analytical gradient is not available when counterpoise is used, in this case the computational cost of these tasks is very high. 6-311+G* or def-TZVP without counterpoise is absolutely fine enough for present task.

Best regards,

Tian

First of all, thank you very much for your outstanding work on MULTIWFN and for managing the forum.

I am currently working on modeling π-π interactions between benzene dimers (Gaussian 16), for example in the “face-to-face” (FtF) geometry as shown in the attached figure. I performed the relaxed scan by keeping the monomer geometry fixed and  optimizing the distance between them. I think I made a mistake because in the route section I used:

opt=modredundant gen nosymm geom=connectivity counterpoise=2 blyp and mpiricaldispersion=gd3 int=grid=superfine

without including the frequency calculation. So, to “fix” the error, I was thinking of saving the minimum geometry obtained from the relaxed scan and calculating the vibrational frequencies based on that with the freq keyword. Is this the correct procedure in your opinion?

If I obtain negative vibrational frequencies, how should I proceed?

Thank you in advance,

Alessio