Re: How to maintain Density constant?

From: Roy Fernando (
Date: Thu Mar 06 2014 - 12:03:37 CST

Thanks Brittany,

Yes, I did. I can see that the system has expanded. But throughout the
system it maintains a uniformity. I mean, the protein sits closer to the
center of the box and then the water and other solutes move randomly. I
transferred the protein to this system after equilibrating in water for
20ns. In this system protein seem to be increasing in size. This is again
an indication of something unphysical going on, because in the new
environment ( which is crowded ) I would expect the system to assume a more
compact structure than trying to be more relaxed.

I am trying out all the possibilities here :).



On Thu, Mar 6, 2014 at 12:41 PM, Morgan, Brittany <> wrote:

> Roy,
> Have you looked at your system visually after the density decreases? You
> would be able to tell, as Ivan suggested, if your system is diffusing
> because the periodic boundary conditions are not set correctly. Also, I'm
> assuming you have a 'typical' system of a protein in water box. Alternate
> solvents, membranes, etc. might have their own special issues.
> Brittany
> ------------------------------
> *From:* [] On Behalf Of
> Roy Fernando []
> *Sent:* Thursday, March 06, 2014 12:28 PM
> *To:* Kenno Vanommeslaeghe;
> *Cc:*
> *Subject:* Re: namd-l: How to maintain Density constant?
> Hi Kenno, Ivan,
> Thanks for detailed explanations.
> While I understand the fundamental physics part, what I was looking for
> is some insights about why my system displaying unphysical (or far from
> being physical) behaviors.
> I am not simulating a gas, I am simulating a protein in a solution. This
> type of a system evolve under atmospheric pressure and therefore choosing a
> NPT simulation makes sense to me. When an NPT simulation is chosen I am
> allowing the system volume to fluctuate to maintain the pressure around
> 1atm. However, what I noticed was not a fluctuation. The density of the
> system dropped by 40% because the system dimensions has increased by 20%. I
> was trying to see if there are other processes that can be applied such as
> maintaining some ion concentration (this system is neutral to balance the
> charges). The density of the solution is expected to be around 1.1 g/cm^3
> but I observed it to be around 0.6 g/cm^3
> Thanks for your help.
> Roy
> On Thu, Mar 6, 2014 at 11:55 AM, Kenno Vanommeslaeghe <
>> wrote:
>> 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
>>> <> 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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