[gmx-users] thickness of interface

Widya Desmarani widya.desmarani at gmail.com
Fri Mar 18 00:00:17 CET 2011

```Dear GROMACS user,

Hello. I am new to molecular dynamics simulation, and also to simulation of
interface. Recently I have simulated liquid-vapor interface of heptane in a
slab with NVT ensemble, and liquid-liquid inteface of water-heptane in NPzAT
ensemble.  I am using GROMOS force field and SPC/E water model.  Using
g_density, I obtained the density profile of each system across the z
direction of my simulation box.  I have two questions I want to ask all of
you.

1.   In GROMACS, how can I calculate the thickness of these interfaces? I
have checked through the manual, and I couldn't find any command for this
purpose.

2.   What is the expression used for calculating the density profile of
liquid-vapor and liquid-liquid interface by g_density? I have gone
through the gmx_density.c code and unfortunately I couldn't understand it at
the moment due to my zero experience in programming.  In a paper I saw that
one author use this equation to determine the coexistence densities between
liquid and vapor interface:
rho(z) = 0.5 [rho(liquid) + rho(vapor)] - 0.5 [rho(liquid) - rho(vapor)]
tanh [(z-z0)/d].
z is the z direction of the box, z0 is location of the Gibbs dividing
surface, and d is the interface thickness.  Is the g_density command also
based on that equation?

Best regards,
Widya
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