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Why are TIP3 LJ-parameters of charmm handled differently?
#7647 08/01/05 05:05 PM
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kokubo Offline OP
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CHARMM seems to handle TIP3 water model LJ-parameters differently from the original model's ones. Hydrogen atom LJ-parameters (epsilon and sigma) are originally zero. Why were they modified? Are there actually any good points to do so? Or Is it an only historical one? Energy is very different, and pressure may be also different from the original model. I feel that the hydrogen's atrractive LJ force influences hydrophobic effects much. Does anyone have clear opinions about this?

Thank you very much in advance.
Hironori

Re: Why are TIP3 LJ-parameters of charmm handled differently?
kokubo #7648 08/01/05 05:36 PM
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rmv Online Content
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I believe the reason is historical, and a number of people have done simulations that suggest the difference is "mostly harmless" with regard to bulk properties of water (density, translational diffusion).


Rick Venable
computational chemist

Re: Why are TIP3 LJ-parameters of charmm handled differently?
kokubo #7649 08/02/05 08:24 AM
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Most properties, including the energy are NOT very different (Eorig-Emodified is less than a kJ/mol; see eg JPC A 105, 9954).


Lennart Nilsson
Karolinska Institutet
Stockholm, Sweden
Re: Why are TIP3 LJ-parameters of charmm handled differently?
lennart #7650 08/02/05 06:21 PM
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On a related note, I'm given to understand that the TIP3 water model isn't as successful at producing the properties of liquid water as it could be, especially at longer ranges, ie

Jorgensen, W. L.; Chandrasekhar, J.; Madura, J. D. "Comparison of simple potential functions for simulating liquid water" J. Chem. Phys 1983, 79, 926-935.

Is this true of the modified model as well? Ignoring the additional computational cost for models like TIP4P and TIP5P for a moment, which water model as implemented in CHARMM best reproduces the properties of water at room temperature? As of 2003, at least, there seemed to be some question of whether even the implementation of TIP4P was producing good properties in CHARMM. How is it now?


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