# Fixing incongruities in PES Scan [closed]

I tried to parameterize a particular x-y-z-t dihedral angle of novel molecule using a Quantum Mechanical scan, but I got the strange result below.

The energy changes sharply around 130° and $$E_\text{dihedral}(0°) \neq E_{dihedral}(360°)$$. I performed PES scan in gaussian09 and psi4 and I got the same results in different softwares.

I'm uncertain what approach to take to fix the scan. I saw some parameterization articles which didn't perform PES scan 0-360 degree. Other authors performed scan in the range of 0-60 degree or 0-120 degree and I didn't predict I will face a result like this. What is going wrong with my scan and how can I fix the issue? Would performing single point energy for each angle instead of PES scan help?

• What are the units on the figure? – Buck Thorn Apr 11 '20 at 19:11
• I'm voting to close this question as off-topic because I'd bounce it back too if I was the referee. The plots should be the same at 0/360 and the discontinuity adds doubt to the methodology. – Todd Minehardt Apr 11 '20 at 22:33
• of course you can vote to close this question... but this is not off-topic. your strange comment is off-topic. Nobody asked you "hey if you were referee what would you do". why are you saying this? Are you trying to get attention? this is a computational chemistry problem (an important one) and we found a solution. TO say "should be equal" is not a solution. You should be more respectful to your olders, teenager. – Barış Kurt Apr 12 '20 at 4:05
• Karsten Theis, Mathew Mahindaratne, Todd Minehardt, Mithoron, Jan,Let me make a criticism, please: It is really funny to mark an article about to be published in an academic journal with 3.5 impact as a "zero-effort homework". lol :) But I respect that. I hope someday you will get nobel prize. – Barış Kurt Apr 19 '20 at 13:07

Even without seeing your molecule, I can say the referee is very likely correct. If you think of doing a scan of the dihedral of hydrogen peroxide, by symmetry the path from $$\theta=180\to360$$ should just be the reverse of $$\theta=0\to180$$. In your case, I don't know the symmetry around the dihedral, but at a bare minimum the energy should be the same after a $$360$$ degree rotation. The fact that it isn't suggests that some scan step (probably that point around $$130$$) irreparably distorted the molecule geometry so that additional steps didn't bring it back towards the initial geometry.