Re: How to maintain Density constant?

From: Kenno Vanommeslaeghe (
Date: Thu Mar 06 2014 - 09:57:24 CST

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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