Dear Mohd,

Once you have find the minimum, you should perform a fine scan around it (this is, with a much smaller step size), with points above and below the equilibrium distance. The region of the PES around a minimum is well described within the harmonic approximation, therefore you only need to adjust this fine scan to a parabola, and the term multiplying x2 will give you k/2.

To get an accurate result, I would like to remind you that the level of theory needs to include dispersion effects (if using DFT, the M06-2X functional should work well), otherwise the result will not be reliable.

Hope you find it helpful.

Many thanks Enrique.

Could you please elaborate more "term multiplying x2 will give you k/2." If possible provide me any literature or web link.

Regarding dispersion effects, I am using WB97XD functional, which is working reasonably good here.

For Benzene dimer, a high-quality scan is available from works of Hobza, dataset S66x8 for example. Your values (I assume in kJ/mol) seem to be significantly overrated. Google images for "benzene dimer potential energy".

U = 1/2*k*(x^2)

U = potential energy (of a spring at a certain position)

k = force constant (also named spring constant, characteristic for the spring)

x = shift from equilibrium geometry (the distance the spring is stretched or compressed away from equilibrium)

That's Hooke's law or "elastic formula". As Enrique Manuel Arpa mentioned - you need part of a curve that can be well described with a parabola.

Many thanks Dmitry.

My system is substituted benzene which is little different from shown in figure.