 After bonds, the second type of interaction we're going to discuss is angles. So I'm using the same type of molecule here, but instead of looking at just a relative displacement in the distance between two atoms, we're looking at how the angle between three atoms is varying. So if you have atoms i, j and k here, we can draw one vector i to j and another vector j to k, and then we're looking at how this angle is varying. This is also a fairly strong effect, just like the bonds, but it's softer than the bonds. And by softer, I literally mean that where a bond would not typically change more than one or a few percent on the room temperature, an angle can change, well, not 10%, but a few degrees definitely. So if a molecule has to be squeezed in or adapt when we're trying to bind something to it, the angles definitely can vary. So we need a reasonable model for the angles. So now that you are skilled physicists and biophysicists, what type of model would you pick for the bond angles? This is a function. It's a function that will obviously depend on this angle and you have to tell the truth absolutely no idea what it is. It's a completely random function that with some sort of complicated shape. And around this, we have some sort of minimum and we would like to model that leads to the vibrations of a few degrees around this minimum. And since you already listened to the bond slide, I hope that you all agree that a reasonably good way of doing that is that we might have a potential that has some sort of force constant multiplied by a different say in the angle squared. We might use a cosine function or something there too. It's not crucial, but it's the first order approximation of how the energy is varying around a local minimum. It works great for angles and now we can describe both bonds and angles.