Water models, numerical simulations questions

In summary, there are many models of water that try to explain different properties, but none of them are accurate enough to explain all properties of water. Most models assume classical mechanics interactions and do not take into account quantum effects, which could greatly improve accuracy. However, simulating quantum effects requires a level of computational complexity that is not feasible for describing a large number of water molecules. Some research has been done on quantum mechanical models of water, but they have not been successful in accurately describing liquid state water properties. The problem is still under active investigation.
  • #1
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From http://www.lsbu.ac.uk/water/models.html, I see that are many models of water that can "explain" one or some properties of water but no model is accurate enough to explain all the properties of water. From what I understand, all models assume classical mechanics interactions (positive-negative charges as for the protons and electrons) with the Coulombian potential and other potentials. It seems that no model takes into account quantum mechanics "effects" like let's say electron tunneling. I know that water molecule has 2 hydrogen atoms and one would expect the quantum effects to be relevant if one had to build a model that can explain the behaviour of water molecules.
I wonder why no model (correct me if I'm wrong) deal with quantum effects? Why are all the models "so bad" at describing water in general? They're only "good" or "so so" in very restricted ranges of parameters like temperature, pressure, etc. Would the models be much more accurate if quantum effects would be taken into account?
Hmm I had lots of other questions but I forgot them :frown:. Thanks for any input.
 
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  • #2
"Properties of water" may include properties of a large number of water molecules, which usually requires to simulate a large number of water molecules, which by mere constraints of reality requires a certain degree of computational simplicity for the description and treatment of the individual molecules. The simulation of quantum mechanical effects in Chemistry usually involves a certain amount of computational effort that does not suit well to the computational simplicity demanded by the requirement to simulate many molecules.

It is not true that there are no models that try to take into account QM effects. Using Google scholar for "water model quantum mechanics simulation" gives me 50,000 hits of which at least some of them seem to be exactly that: the description of a simulation of water that takes into account quantum mechanical effects. Note that usually QM simulations are also approximations, and will certainly not take into account all in principle possible degrees of freedom.
 
  • #3
In addition to what Timo described, you will be surprised to know that the classical water models (SPC, TIP5P,...) do a much better job compared to quantum mechanical models (DFT, Hybrid DFT+HF,..) in terms of describing bulk liquid state water properties. Of course this is not because water is well-described by classical mechanics, it seems that there is significant error cancellation that makes these models work fine. Understanding why quantum mechanical models fail to describe liquid state water is an active research area.

On the other hand, gas phase water or let's say the single water molecule properties are more or less well-described by quantum mechanical models. This indicates that the problem is indeed in describing water-water interaction.
 
  • #4
Ok thank you guys.
Finding a good -in that it gives several if not most properities of water quite accurately- liquid water numerically applicable model looks a really nice and interesting challenge.
 

1. What are water models and why are they used in numerical simulations?

Water models are simplified representations of the physical and chemical properties of water. They are used in numerical simulations to predict the behavior and interactions of water molecules in different environments. This allows scientists to study the properties of water in a more controlled and efficient manner than in real-world experiments.

2. What are the most commonly used water models in numerical simulations?

Some of the most commonly used water models include the TIP3P, TIP4P, and SPCE models. These models differ in their level of complexity and the parameters used to describe the interactions between water molecules.

3. How are water models validated in numerical simulations?

Water models are validated by comparing the simulation results with experimental data. This includes properties such as density, diffusion coefficients, and radial distribution functions. If the results of the simulation match the experimental data, it is an indication that the water model is accurately representing the behavior of water.

4. Can water models be used to study other molecules besides water?

Yes, water models can be adapted to study other molecules such as organic solvents, ions, and proteins. However, it is important to ensure that the parameters used in the model are appropriate for the specific molecule being studied.

5. What are the limitations of using water models in numerical simulations?

Water models are simplified representations of the complex behavior of water molecules, so they may not accurately capture all aspects of water's behavior. Additionally, the accuracy of the results depends on the quality of the parameters used in the model. Therefore, it is important to carefully select and validate the water model before using it in numerical simulations.

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