You can use Dmol3.

In most QC and MM packages, you can keep the distance between selected atoms fixed while optimizing the remaining structure. In order to calculate the dissociation curve, you just need to increase the distance in your starting structure between the atoms you're interested in. At some distance the energy will reach a horizontal asymptote. The difference between this asymptote and the equilibrium structure energy is the dissociation energy.

If you just need the energy, you can just place the fragments at a very long distance.

Please be aware of some issues: if you form radicals, the energy at long distance needs to be calculated using an unrestricted calculation:

https://i.stack.imgur.com/oGTtD.png

There may be convergence issues, in Turbomole that can be fixed with the "Fermi smearing" algorithm, maybe Materials Studio also offers that?

using Hartree Fock theory

Hadi Jabbar Alagealy : why should she use Hartree-Fock? A well-chosen functional+basis set give more accurate results and, depending on the choice, the DFT calculation may run faster?

I have some issues with installing version 17. Previously i was using version 8.0 When I run my calculation, a dialogue box open saying that spin orbit interaction and Ultra soft pseudo potential not supported. Can you suggest me what to do?

Fabiha Faiza you should switch to norm-conserving pseudopotentials. Is this with CASTEP? Version 17 is rather old, I recommend upgrading to CASTEP 20. CASTEP is available for free for academic research:

https://licenses.stfc.ac.uk/product/castep

If you're using Materials Studio, you should be able to upgrade the CASTEP "in place", but of course the actual Materials Studio interface will remain at whichever version you purchased.

Thank you so much!