Dear Ahmed,

Please look at similar Questions trough ResearchGate here is folowing links :

- What is the simplest way to calculate Basis Set Superposition Error ...

https://www.researchgate.net/.../What_is_the_simplest_way_to_cal...

- Is geometry optimization needed before running single point energy ...

https://www.researchgate.net/.../Is_geometry_optimization_needed...

- How to use counterpoise correction method along with periodic ...

https://www.researchgate.net/.../How_to_use_counterpoise_correct...

Also look at link and attached files in topics.

- How is a counterpoise corrected geometry optimization done?

chemistry.stackexchange.com/.../how-is-a-counterpoise-corrected-...

 Best regards

Dear Ahmed,

I think the manual of ORCA explains this process in much details, if this was the first time to do that, I will do it in ORCA and if i need Gaussian, I will follow the same protocol.

https://orcaforum.cec.mpg.de/OrcaManual.pdf

The process is outlined in page: 78

Regards,

Marawan

read ur querie,

to ur optimised file use the keyword for bsse correction and after the file gets opt read the ouput file and u wil get the bsse correction value. plus we mostly use bsse correction if ur using two systems interaction . single point is used to find the relative dist it gives us energy only that cant be used for opt, so opt ur complex with input of freq and opt then reun it with bsse correction. use can read my paper ie theoretical studies on niobium complex that can help u to understand it better. hopr ur querie is answered. 

After optimisation

1st : optimization    (opt and freq)

2nd Counterpoise calcul applied to final geometry

Counterpoise corrections for BSSE are usually used in a single point calculation, not during geometry optimizations. There are 2 justifications for this. 1) Generally the CP correction has a small effect on the geometry (I have no reference for this off hand, but it's a common justification in the field of intermolecular interactions). 2) Applying the CP correction to the optimization is very computationally intensive. This is because a counterpoise-corrected calculation of the energy actually involves 5 separate energy calculations, and when this must be done at every optimization step, it is too costly.

single point calculation after optimization of patterns

I feel that geometry optimized on the potential energy surface corrected for BSSE or counterpoise corrections would lead to more accurate results. Also, this would be more crucial in cases of molecules where dispersion and induction interactions are more dominated. Certainly, such calculations are expensive so one can balance the accuracy with the computational cost.

Is it necessary to do it in Gaussian? If not then it is easy to do it in GAMESS (US), Firefly package. You can follow the procedure explained in the link below

Article Cluster ions in saturated vapor over barium difluoride: Stru...

Thanks a lot, everybody. This was enlightening.

Good luck!

I think a CP correction "during" geometry optimization is meaningless. CP corrects for artefacts in energy differences due to incompleteness of basis set. The correction should not affect the final optimized geometry of the cluster.

First of all, thanks ya doctorna Hossam for the answer. I really miss you.

But I guess if you allow the basis sets to overlap at small distances this would produce more favorable geometry at small distances than it should actually be. Thus the molecules might prefer to be closer to each other than they should be. I'm not sure but this is how I view it.

CP correction doesn't add any basis functions to the cluster, as far as the latter is concerned nothing changes with CP, neither the energy nor the forces.

I have the same question if you could find the answer please let me know

You basically do it after optimization, not during it.