 We are to bring all these interactions together. This is another beautiful illustration that I got courtesy of Mike Levitt many years ago. Don't worry too much about the exact shapes in here, because again, different programs, different scientists tend to use slightly different standards for this. But if I want to describe something in a molecule and describe the complete potential, there will have to be one term that corresponds to all these harmonic bond vibrations. Then we're going to need one term describing all these torsion potentials, and that's the one in the middle here. I can't stress enough how important torsions are, and the reason why torsions are important is that they're intermediate. You won't know that yet, but it's going to turn out that energy variations that are very small, that's kind of like gravel in the road. We run over them, it vibrates a bit, but it's not really going to change where we get. And it's going to turn out that energy variations that are very small, change where we get, and energy barriers that are very high, they're going to be like brick walls, you're not going to get through them. So the interesting aspects are the middle, the average energy barriers, and that's exactly the torsions. And again, they're illustrated by these rotations around the central bond. Then you have these Lenard-Jones interactions. Again, this is formulated in a slightly different way, where we have a unit of energy and then an equilibrium length there of the bond. It's a small mathematical exercise to show that this corresponds to those C6 and C12 numbers I had on the last slide. And that would correspond to this repulsion and dispersion part. And finally, we have an electrostatic energy where this has to do with units, whether you're 104 pi epsilon zero, or whether you're using CGS units. So forget about the 332 factor there. Those are the electrostatics, hundreds of k-cals. This might be 0.1 k-cal, torsions, a few k-cals, and these are hundreds of k-cals too, and we're never going to be able to break them. So in principle, that's everything you need to do to describe what the energy is in one confirmation of a micromolecule. But here's the thing. The interesting part is we want to compare different confirmations, what happened when they move. And now things are going to start to be quite different in quantum and chemistry, because this is much more difficult for quantum chemistry to do than for us. So imagine that we take all these things together and sum them up. We're going to come back later to how you let a computer do this, but if I have a potential, if you've studied your undergraduate physics, you know that the negative derivative of the potential that's going to correspond to the force on a system. Again, the potential of lifting a weight is the mass times the gravitation factor times the height, and the force down is the derivative with respect to the height minus the derivative. If we know the force on molecules, we use Newton's first law. If I know the force, I can calculate the acceleration. Force equals mass times acceleration. And the acceleration describes how the velocity is changing. So if I started from some velocities, for instance, zero, I can then calculate how the velocities would be a very little while later. Then I know what the velocity is. And the velocity is how the position is changing as a function of time. So that means that I can calculate how the positions are varying. So this enables me to take small steps and really simulate in a computer how a molecule would move as a function of time. The problem is that small is something that should be taken literally. You're going to need to do this in orders of femto seconds. So this is just one more molecule from the simulation I previously showed you a couple of times moving. And I've hidden all his neighbors in this case. But the advantage is that computers can do 10th of thousands of these steps per seconds today. And we have very large supercomputers can treat molecules with millions of atoms. So this has suddenly become a very useful biochemical tool. Whereas 20, 30 years ago, it was something that only theoretical physicists used for very simple systems. What can we do with that? Well, let's see if we can revisit some things such as how large molecules move or first made the hydrogen bond.