# Re: How to maintain Density constant?

From: Kenno Vanommeslaeghe (kvanomme_at_rx.umaryland.edu)
Date: Thu Mar 06 2014 - 10:55:28 CST

It's perfectly normal (and indeed unavoidable) in constant pressure
simulations for the volume (and hence density) to equilibrate a bit, then
fluctuate. As long as the change is not catastrophic (which was not
indicated in Roy's e-mail), there's nothing to worry about. If the density
is really important, one can run NVT, but then one has to give up control
over the pressure.

On 03/06/2014 11:34 AM, Ivan Gregoretti wrote:
> Thank you Kenno.
>
> So, going back to try to help Roy.
>
> 1) Running MD simulations where you control both pressure and
> temperature is routine. Kenno tells us that it makes no sense to also
> try to impose a control on the volume. (Of course.)
>
> 2) Why is your system's density dropping? I wonder if the periodic
> boundary conditions are properly set. I can picture a situation where
> there is no boundary conditions and the molecules start to slowly
> diffuse away from the center of mass of your system.
>
> Ivan
>
>
> Ivan Gregoretti, PhD
> Bioinformatics
>
>
>
> On Thu, Mar 6, 2014 at 10:57 AM, Kenno Vanommeslaeghe
> <kvanomme_at_rx.umaryland.edu> wrote:
>> On 03/06/2014 08:23 AM, Ivan Gregoretti wrote:
>>>
>>> p V = n R T
>>>
>>> with n being the number of molecules and V being volume. It's an ideal
>>> gas state equation. Notice that n/V is your density.
>>>
>>> In your molecular dynamics, n does not change, so, if you want to keep
>>> the density constant, you need to run your simulation at constant
>>> volume.
>>>
>>> Do I get it right Kenno?
>>
>>
>> Mostly. We usually don't simulate gases, so the ideal gas law you brought up
>> is of very limited value, but there exist similar equations for liquids and
>> solid, and one thing they all have in common is that (assuming n is
>> constant) out of p, V and T (and also E and/or Q), you can set two to an
>> arbitrary value, but then you don't have control over the other one(s); this
>> is very fundamental and doesn't take advanced statistical mechanics to see.
>> There exist mechanisms in nature to impose *some* combinations of these
>> variables on a system, and these combinations ("ensembles" in thermodynamic
>> speak) are often implemented in MD engines. The "constant pressure and
>> volume" Roy asked for is not one of them; even if someone would somehow
>> implement it, it would be of no practical relevance. Besides, the
>> temperature would shift and fluctuate uncontrollably (remember, you can only
>> choose 2), which is probably not what Roy (or anyone else) wants.
>>

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