# [gmx-users] [Fwd: some questions about simulate a liquid-vapor interface of a solution, many thanks]

Mark Abraham Mark.Abraham at anu.edu.au
Thu Jun 12 18:33:31 CEST 2008

```Mark Abraham wrote:

> Hi, Mark
>     Thank you very much to your reply. I cannot undersand some points in
> when the system is equilibrated?

As I said last time, "Trial and error might be the best approach". You
can make a guess in the following way. The total volume is the volume of
the solution and the volume of the vapor. For a given number of vapor
atoms at known temperature and pressure you can use the ideal gas law to
find the gas volume. For a given amount of solution you can determine
the volume from the density. This lets you choose V as as function of n
- though how you choose a partition of n over the two phases is your
problem!

> in addition, do you mean the sysytem is
> sandwich box or the cubic box?

I didn't mention a box, and under PBC whatever distinction you're trying
to draw is probably irrelevant.

Mark

>     Best wishes.
>
>
> Beibei
>
> beibei wrote:
>> Hi all,
>>        I want to simulate the liquid-vapor interface of I lithium
>> bromide solution at 303K and 0.008bar. First I put the solution to a
>> cubic box and used NPT ensemble to simulate. Then, I enlarged the box
> to
>> form two liquid-vapor interface,and used NVT ensemble, but the
> pressure
>> is not 0.008 bar.
>
> You need to determine a volume that will lead to this pressure when the
> system is equilibrated. Then you need to measure for long enough to get
> a meaningful average pressure. Trial and error might be the most
> effective approach. Best of luck!
>
>> If I used NPT emsemble in the second step, the box
>> becomes smaller with the run time.
>
> Of course... you won't get phase separation at equilibrium in NPT
> unless
> you happen to be at a critical point. Think about heating and cooling a
>   closed and open beaker of real water.
>
>>  I want to know how to do when  I
>> want to simulate the liquid-vapor interface of a solution at a low
> pressure.
>
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