Melting point of gas hydrate

LAMMPS LAMMPS General Discussion
1

Hello

I am trying to find the melting point of gas hydrates in a certain pressure (solid made from water+gas). How can I do it using LAMMPS? One way to do it is to try different temperatures in NPT until I find a narrow range of T in which the solid melts, but I think there should be an automated method to find the Tmelt. Any suggestions?

2

There are multiple previous discussions on determining the melting point from MD simulations. There is also published literature.

That is not likely to work. Melting (and freezing) are activated processes and thus have a hysteresis which can be very large at the atomic scale (you can get supercooled liquids).

1 Like
3

Thanks, Axel.

How about melting point estimations using free energy calculation? Is there a command in LAMMPS that can straightforwardly calculate the free energy?

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Free energies are always associated with a specific process, so you would have to define something like a collective variable that correctly describes the melting/freezing process. That could be quite tricky.

What I know to work are coexistence simulations, i.e. you set up a solid system, then melt half of it and equilibrate each half separately to the expected melting temperature. From this equilibrated configuration you now start multiple simulations where you thermalize the entire system with one fix nvt instance at different temperatures near the expected melting point, and the one where size of the molten/frozen part does not change is at the melting point. If below the frozen section will grow, if above the molten. There is plenty of published literature on that method. Its been used for a long time. the main challenge is to define a computation where you can easily determine whether an atom is frozen or molten, but LAMMPS has a large variety of such compute styles and you just need find the one that is the most suitable.

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5

Thanks Axel.

Finally, I did the following procedure to estimate the melting point temperature:

  1. put hydrate crystals within the box and set T and P to the hydrate formation conditions in NPT ensemble
  2. Did NVT only on half of the box with T>Teq , so half of the box melts down
  3. Then do NPT (with T=Teq, Teq is the temperature we are trying to monitor the interface for) over the whole simulation box.

Now, my question is related to step 2. When we implement NVT only on half of box, the other half will remain fully intact? I mean what will happen to the molecules on that half ?

I tried to implement NVT on the whole box but keep half of box frozen, but I faced the error telling some molecules or atoms missed.

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That is what we have visualization software for.

Then you probably did not do it correctly.

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Good point :slight_smile: The visualization of the results using VMD shows no noticeable changes at the interface, yet I wanted to make sure about the accuracy of my implementation since I will repeat the same procedure for various different temperature for which I might not be able to visualize all of them.

I guess the huge repulsive forces at the interface could have caused it but I don’t know how to avoid that. Thanks for your response anyway!

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NEVER guess. You can check this. Speculation is almost always the first step on a path to an incorrect interpretation of your data.

If you set up the dual phase system correctly, there will be no problems at the interfaces (remember your system has two!!), because each half of the system will be properly equilibrated (individually).

This discussion is now rapidly moving far away from discussing LAMMPS and becoming a discussion on how to do research with MD simulations. That is a discussion you need to have with your adviser and colleagues. This forum is not a suitable place and I for one simply don’t have the time and patience to give you a step by step tutorial, and point out all your mistakes and misconceptions to you via a forum like this (which is an extremely inefficient way of communicating such matters). As I already mentioned, there should be many publications describing how to setup, perform, and analyze coexistence simulations. So please help yourself and your understanding of this method by having a good read of a representative sample.

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Thanks, Axel, but I did not ask any questions in my previous reply! I simply shared my own thoughts…

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My remarks were not only in reference to the very last of your responses, but also to the ones before and the general direction of the discussion. I have the impression that you have glossed over a significant chunk of my previous recommendations and thus wanted to remind you of them and also reiterate what you can expect from a forum like this (and what not).

BTW: I am just sharing my thoughts, too. :wink:

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Thanks, Axel. If you found my questions irrelevant/inappropriate simply just ignore them! This way you will save your time and mine, as you stated:

I am sure there will be other experts with more patience to engage in the discussion.

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Can’t do that. We want the discussions in the LAMMPS categories to stay on-topic. If you want to discuss with people on more general terms, you should post in a suitable general category like “Science Talk”

The alternative to pointing this out would be to flag the problematic post(s) as off-topic and then nobody will see them. Please also keep in mind that replies are not only directed at you personally but in addition to people reading/searching through old topics.

Please just move the discussion to a more suitable category.

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Tried but did not find out how to move the topic to another category. You are welcome to label it as off-topic. Most of the messages here are ethical advice anyway, so I am going g to create a new topic and put it under the “Science Talk” category.