 So on the one hand, it's great that we can understand interactions because they're due to electronic interactions. The problem with electrons is that we cannot treat electrons classically. There is no way we can treat electrons with simple ball and stick models. So does that mean that we should give up on this class and head over to the quantum mechanics class instead? Well, yes, in many ways we should, right? Because if it is about electronic interactions, we, I just spent a few slides or boards here justifying that we need quantum chemistry, so duh, of course we need quantum chemistry. You could even argue that I started introducing you to quantum chemistry even if we didn't use equations for it. But on the other hand, we have all these simplified models that I showed you that are literally ball and stick models where we think of an atom as a ball and bonds as sticks. And people keep using them all over the world. So why do that? Does that work? Well, you could argue that either that you're a realist or you don't know what you're doing. There are many reasons why people have done this historically. Today we can treat maybe 100 atoms really accurately with quantum chemistry when I was a student with probably six or so. But that is not an excuse. It's not an acceptable excuse because it's, if it's expensive or difficult to do it right, does that mean that it's going to work to do it wrong? So I think we'll have to strike out that argument. The other problem though is that what I have here on the other side, that's not really accurate either. Because what you do in quantum chemistry is that you put things in a computer and then you calculate what is the best possible distribution of electrons? Well, first the best possible one. We are working with time in this class so that you would have to solve the time-dependent relativistic Schrodinger equation. And you can probably do that for like one electron. The other problem is that if you determine the best distribution, you're essentially determining what is the structure at zero kelvin. And I hate to break it to you, but there's not a whole lot of interesting biology happening at zero kelvin. The other problem when you do this, take any small molecule in life science, that's not going to work. We need water. I spent a large part of last lecture talking about the virtues and importance of water. You can't start doing life science, but say that you're going to approximate everything being at zero kelvin and no water. That is just as stupid as the first item here saying that you're going to do a bad method because the usual method doesn't work. So it turns out that quantum chemistry on the one hand has a very accurate representation of some things, the energy between the atoms that are included, but it has a lousy representation of other things. It doesn't represent motion. It doesn't represent the solvent and it can be surprisingly difficult for it to get very large systems to work and the time dependence isn't there. The classical models on the other end have other strengths. They are simple so that they're very fast to calculate. That means that we can reach realistic sized systems. We can reach systems that are in the water and we can simulate those systems at realistic room temperatures when things actually move, so that we reproduce what happens, for instance, if two atoms, not two atoms, but two molecules get close to each other or if a protein is folding. And that's going to be a huge difference that we're going to come back to in this class.