Computational Chemistry 2.7 - MM Torsion Strain Terms

Short lecture on torsion strain energy terms in molecular mechanics.
For each torsion angle in molecular mechanics, the energy term is a sum over periodicities (1 through 6) of the rotation barrier times 1 plus the cosine of the n-foldness times the torsion angle minus the phase offset. Each term of each torsion requires 4 parameters.
Notes Slide: i.imgur.com/Vy2IGAt.png
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[@miracle2660]

This is amazing, it's helped me understand these concepts so much better. Thank you so much for making the series!

[@mrbrohere]

3:47 it's 'Anti conformation'

[@solsticetwo3476]

Great series man. Thanks

[@TMPChem]

Thanks for watching.

[@antonychipman3088]

positive Gauche; anti; negative Gauche. cheers

[@AvengedNoun]

Hello, thank you so much for your series. Can you please explain me the meaning of phase offset?

[@amritasharma3392]

Can you explain why n=3 and when does it vary?

[@TMPChem]

n = 3 is typically what you will see in torsions around consecutive sp3 carbons. Ethane, for example, has 3 hydrogens emerging from each carbon. If you maintain constant bond lengths and bond angles, then there are two types of configurations: staggered, and eclipsed. Every 120 degrees of torsion rotation, you go from staggered to elipsed and back to staggered. 360 / 120 = 3, so the torsion potential of the hydrogens is 3-fold periodic. If you move to butane, you now not only have H-C-C-H torsions, but also a C-C-C-C torsion. The staggered conformations of this torsion can be gauche or anti. To replicate that type of potential takes a 3-fold and a 1-fold term. Typically terms are only included as needed, and 3-fold is by far the most common, with 1-fold being the second most. There are also ocassionally 2-fold and 4-fold terms as well. I don't think there are any 5-fold terms in the AMBER force field.