Cavity size (Technical)

So I've just finished some calculations that took up a fair amount of computing time of Raijjin. But which came to basically nothing as I just proved my hypothesis wrong. This isn't totally a bad thing as I can rule it out as a possibility. But I can't help but wonder whether someone else has had the same idea tested it realised it didn't work and therefore abandoned it, and I never heard about it as there was nothing to publish. So I ended up wasting my time by checking it. I guess this is a similar problem to what they have in Medicine and Psychology where negative trial results just sit in the bottom drawer and never get published.

It's a little different I guess as there's no way I could write up and publish every idea I had that came to nothing, it would make reading journals a nightmare sorting through all the things people had tried. I guess we could do with some kind of online wiki article where we all describe things we tried that didn't work to solve a problem so others know not to try it as well.

I might as well describe the idea. It was that as two ions come together their electric fields will partially cancel this will mean there will be a weak ion-water electrostatic interaction. The water molecules will therefore move away from ion. This will reduce the ionic solvation energy leading to a repulsive force. So I performed Quantum Chemical geometry optimisation calculations on a sodium chloride dimer with 10 water molecules around them. But in the end the water molecules barely budged at all from where they were around the single ions. I also did MD simulations around the dimer and saw that the peak in the RDF stayed in exactly the same position around the dimer. Although it lowered a little as you'd expect due to the other ion removing some water.

On the plus side I learned the basics of performing MD. Was kind of ridiculously easy, the only difficult thing pretty much was getting the files in the right format. And as a result I got vast amounts of information. I can understand why it is so widely used so easy and satisfying. The only problem is that I barely knew what to do with all that information. It was kind of overwhelming. I had a simple hypothesis that I was testing and it was great for that. But if I was just trying to understand the problem more generally I wouldn't know where to start. I get that feeling a lot reading some simulation papers, that they just throw all this information together then kind of give hand wavy interpretations of what's going on and then write it up. And it's not really clear at the end of it what's been learnt.

Polarizable Water Models (Technical)

One frustrating thing is that I often have ideas about stuff that I can't write up as a paper because it isn't substantial enough. Because theirs no real new results or theories I have its just opinion and speculation. So I figure I could put it here, so its recorded.

So one thing that puzzles me a lot about Molecular Dynamics water models is that as far as I'm aware no one has built one that incorporates induced multipole moments higher than the dipole moment.

This doesn't make sense to me, it has been shown that the electric field varies substantially over the range of a single water molecule.   That means it will have significant derivatives and hence significant induced quadrupole and octupole moments. There are models that use Drude oscillators but this is not the same thing as I'm pretty sure these will be dominated by the dipole moment as the point charges are so close together. 

I think this causes the common problem with water models. That the polarisability has to be artificially reduced from 1.44 cubic angstroms to about 1 as otherwise the dielectric constant of water comes out too large at around 100. (paper here) This paper argues that on average the electric field is lower around a water molecule than it is at the centre, and that that is why you should lower the polarisability. But strictly speaking the correct way to account for that is to use higher order induced multipole moments as you are taking a taylor expansion about the centre of the molecule. Additional evidence for this comes from a relatively newer water model which has Drude oscillators on the hydrogens as well as the oxygens. This model seems to able to reproduce the dielectric constant of water with the vacuum polarisability value of 1.44 angstroms. That implies that it is the variation in the electric field that explains this effect, and the correct way to account for this is with a multipole expansion. I guess it's too computationally expensive though, although it looks like a quantum drude oscillator model might work.

They also apply their model to ion solvation energies and get nice results for the salt pairs. But they don't calculate the surface potential of their model. So there is no way to know what they're getting for the real or intrinsic values. Their single ion values in periodic boundary conditions are really close to Tissandier's values, which is surprising but I guess you can't read too much into that as the quadrupole trace of their water molecule could be anything. Surely it's not too hard to calculate this, would be really useful for working out what the real and intrinsic values are, which is super important. 

Here's a video of my 3 minute thesis talk I gave at the ANU finals.