[gmx-users] lipid monolayer isotherm

[Hector Mtz-Seara]

Thank you for you quick answer because I've started to become crazy. 
I'll see more carefully your preparation of the system( to be honest I 
don't understand it) , but I can't see why it shoud be faster to reach 
the equilibrium or even reach it and my way or preparing the system 
isn't a properly way. There's any explanation?
Do you know where I can find a properly and well known arametrized fatty 
acid for make some tests?

Thanks again. Hector

>On Tue, 18 Jan 2005, Hector Mtz-Seara wrote:
>
>sorry Hector, my mistake: g_energy actually displays surface tension,
>the equation I referred to is meant to convert surface tension into
>surface pressure, which is the most common way to present an isotherm
>(surface pressure against area per molecule). anyway, a surface tension
>larger than that of water means that your system is not equilibrated
>yet (it gives a negative surface pressure).
>
>  
>
>>Sorry after reading again and again there is one question I'm not sure
>>about.
>>
>>    
>>
>>><>
>>>g_energy output gives surface pressure, not surface tension.
>>>      
>>>
>>I've been reading the gromac's manual an in 7th character , page 138 ,
>>first paragraph, it speaks about surface tension, not surface pressure.
>>In the option list of g_energy also speaks about #Surf*SurfTen. Then why
>>you say the output is the surface pressure?. If you are right then where
>>is the value of the surface tension of the water to make this output in
>>the gromacs program. By the way it then means that you can only
>>calculate "surface pressure" for monolayers in pure water systems?. Can
>>you explain me that a little more.
>>    
>>
>
>no, it means that you need to know the surface tension of the subphase,
>whether it is pure water or any other substance or even a solution.
>
>  
>
>>><>to compute the <><>surface tension you have to use the standard
>>>expression:
>>>
>>>gama = gama_0 - pi
>>>
>>>where:
>>>
>>>gama = surface tension
>>>
>>>gama_0 = pure water surface tension
>>>
>>>pi = surface pressure
>>>
>>>anyway, if you're getting surface pressures 3-fold larger than that
>>>of pure water, then your surface tension is negative, isn't it? if
>>>that happens to be the case, then your systems may be still away
>>>from mechanical equilibrium.
>>>
>>>maybe you could give some more details of your simulations: how did
>>>you assembled each monolayer? how long was each simulation run? if
>>>you plot surface area along the simulation time, is it stable?
>>>      
>>>
>
>
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>    Dr. Andre' Farias de Moura
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