# Zero Surface Tension

• sdf
• Topic Author
• Offline
• Fresh Boarder
9 years 7 months ago #521 by sdf
Zero Surface Tension was created by sdf
I just want to make sure I am getting this. When I read papers on lipid bilayers, they often simulate at "zero surface tension". For this, I have been using

Pcoupltype = semiisotropic
ref_p = 0.0 1.0

because the GROMACS manual says that
"""The first ref_p value is the reference surface tension times the number of surfaces [bar nm], the second value is the reference z-pressure [bar]"""

However, after seeing the standard MARTINI mdp options file...
md.chem.rug.nl/cgmartini/images/paramete...ini_v2.x_example.mdp

I am beginning to wonder if I should simulate zero surface tension with pressure coupled separately at 1.0 bar in each direction.

So, can someone please confirm that I should be using

Pcoupletype = semiisotropic
ref_p = 1.0 1.0

and maybe explain why you wouldn't use 0.0 for the first coupling value.

Also, while I am at it, is there any difference between Pcoupletype "semiisotropic" and "surface-tension?"

Thank you for any help.

9 years 7 months ago #568 by xavier
Replied by xavier on topic Zero Surface Tension

sdf wrote: I just want to make sure I am getting this. When I read papers on lipid bilayers, they often simulate at "zero surface tension". For this, I have been using

Pcoupltype = semiisotropic
ref_p = 0.0 1.0

This would have you to simulate a system with a zero pressure on the x/y plane!

because the GROMACS manual says that
"""The first ref_p value is the reference surface tension times the number of surfaces [bar nm], the second value is the reference z-pressure [bar]"""

gromacs says this indeed but in the section "surface-tension" of the pressure coupling options.

However, after seeing the standard MARTINI mdp options file...
md.chem.rug.nl/cgmartini/images/paramete...ini_v2.x_example.mdp

I am beginning to wonder if I should simulate zero surface tension with pressure coupled separately at 1.0 bar in each direction.

So, can someone please confirm that I should be using

Pcoupletype = semiisotropic
ref_p = 1.0 1.0

and maybe explain why you wouldn't use 0.0 for the first coupling value.

Also, while I am at it, is there any difference between Pcoupletype "semiisotropic" and "surface-tension?"

I believe you should read the pressure control in the gromacs manual.
Thank you for any help.[/quote]

• sdf
• Topic Author
• Offline
• Fresh Boarder
9 years 7 months ago #601 by sdf
Replied by sdf on topic Zero Surface Tension
You are completely right. I dont know why, but I guess the two pressure options caught me off guard. Turns out that I was specifying the correct pressure though. Thanks for helping get that cleared up.

Now, if I can only nail down the compressibility issue...
md.chem.rug.nl/cgmartini/index.php/user-...rm/forum/topic?id=62