 the knowledge now about the typical interactions we have in atoms and having a little bit of grasp about how atoms move, we're now going to revisit the hydrogen bond. It's not going to be the last time we revisit the hydrogen bond. Remember that simple atom, oxygen and hydrogen water, the miracle of life. Well, let's be proper. Instead of doing those nasty partial charges, let's assume we had an infinite amount of computing power and that we were to calculate the exact electron density around this. If we were to do that, you're going to need to trust me that we would have higher electron density around the oxygen, red here and low electron density around the hydrogens. This could only be described with a three-dimensional density function describing where it's high probability of finding electrons and whether it's low probability of finding them and then we'd have to visualize that in some part. But since I need something simple, what I'm going to take any the electron density that is closer to the oxygen, I would just assume that that is placed on the oxygen and that's when we can do the things that we pretend that we have minus say 0.82 is one common water model, sorry, and then plus 0.41. I almost said that minus 0.82. That's what I call a partial charge. It's really a pure invention. I typically need the quantum chemistry to calculate this, but this is now a much simpler model where I can apply ball and stick and just use traditional Coulomb interactions. For something as simple as water, I could theoretically parameterize this based on an experimental result and that typically how we do it for water models. But if you have something that's part of a drug molecule or something, the only way to do this is put this into a computer and run a quantum chemistry program to try to find what is roughly the partial charges for this particular molecule in isolation. Again, this is an approximation. There is no question about it, but by doing this approximation, we're going to be able to handle all these other aspects that I just took you through. We're going to be able to handle flexibility in this molecule. Every torsion here can move. We're going to be able to handle what happens when it's interacting with other molecules and what happens if we put it in water so that we pay some, but we gain tons of other things. That's roughly where we're increasingly going to be heading. We're not going to spend the majority of this class doing computer simulations, but this type of sequential modeling is really going to help us understand what happens inside molecules. We already introduced hydrogen bonds a little bit. If you just let me erase these things here, we can start to think what's happening inside a