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Further information in: The MARTINI Force Field
- panzu
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I am wondering if you could clarify me the next question:
The TABLE 4 of the paper, The MARTINI Force Field: Coarse Grained Model for Biomolecular Simulations,reports results about the Surface Tension of the water/vapor, dodecane/vapor and the Interfacial Tension between dodecane/water.
There is data about the system such as: Small system (400 CG beads per solvent phase) and large system (1600 CG beads), but in fact, it is not as important the number of CG beads of the system as the size of the system.
Therefore, I would like to get further information, in particulary, I would like to know how large the simulation box is for the dodecane/water interface.
Thanks in advance!
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- jaakko
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I don't really understand your question. The number of beads of solvent directly correlates with the volume of the system in constant pressure. A few tens of ns of equilibration of a solvent box with 400/1600 beads should give you your answer. Just build the system and try, you'll have your answer after a coffee break.
Just to give you an idea of the length scale, the equilibrated box of 400 CG waters in this website is a cube with each side about 3.6 nm.
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- panzu
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Neither I think you would get bulk property with such as small size...
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- jaakko
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Emphasis mine, quote from the 2007 Martini paper.In order to study size effects, both a small system (400 CG beads per solvent phase) and a large system (1600 beads) were simulated. Simulations of 1 μs proved long enough to accurately calculate the interfacial tension. The results are summarized in Table 4. Finite size effects are actually important when calculating the interfacial tension. The tension is systematically smaller for the larger system size. We attribute this to the development of capillary waves which are supressed in the small system. Additional simulations for even larger systems show no further decrease of the measured tension.
Hope this helped with your question.
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- panzu
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Thanks a lot.
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- panzu
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The paper says that the working temperature is 293 K. So I believe you used antifreezen water in the water slab in order to avoid the solid state, right?
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- panzu
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- xavier
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You have to be careful on the definition of sigma=0.47 nm. It is the minimum of the potential and not the crossing with the Epot=0.
The us of a shift function of the LJ potential also modifies the potential. That might need to be taken into account also. You can try to print the actual potential within GMX.
panzu wrote: In fact I do not understand why your water is freezen between 280 K and 300 K. If you plot the L-J diagram for water with epsilon=5.0 kJ/mol and sigma=0.47 nm and truncate it at 2.5*sigma you find the solid state at 1 bar from 330 K to backward
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- panzu
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What I had ploted was the phase diagrams of the shifted and truncated LJ at 1.175 nm epsilon=5.0 kJ/mol (of water) Maybe that is why I got a larger freezen point.
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- xavier
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panzu wrote: What I had ploted was the phase diagrams of the shifted and truncated LJ at 1.175 nm epsilon=5.0 kJ/mol (of water) Maybe that is why I got a larger freezen point.
Not sure what you mean here ...
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- panzu
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I plotted the phase diagriam (melting line or freezing line in a P-T plot) and using a shifted LJ at 1.175 nm and epsilon=5.0 kJ/mol I get that the water is frozen at 330K while in your model the water freezes between 280K - 300K.
Am I missing something?
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- jaakko
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panzu wrote: well, I mean how the phase diagram for water in your model looks like.
I plotted the phase diagriam (melting line or freezing line in a P-T plot) and using a shifted LJ at 1.175 nm and epsilon=5.0 kJ/mol I get that the water is frozen at 330K while in your model the water freezes between 280K - 300K.
Am I missing something?
Hi panzu,
it's hard to say what's the difference since I don't know how exactly you calculate the phase diagram. Obvious differences are that you say you shift at 1.175 nm (2.5 sigma). I don't know why you do this but assume it has something to do with your calculation. So there's clearly differences between how the shiften LJ in Martini (as implemented in GROMACS) looks like and what you calculate. Also note that the shift in GROMACS isn't just a shift of the potential. Another thing that I think I mentioned already earlier is the artificial ordering due to periodic effects. I have no tangible to show what the effect of that on freezing point is but I'd hazard a guess you'll see a significant difference in the freezing temperatures if you compare very small boxes of water to large ones.
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- panzu
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Thanks!!
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- xavier
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panzu wrote: Ok, I see that the difference does not only come from where I shift the LJ as I suspect.
Thanks!!
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- Pim
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