The use of NPT or NVT ensemble to observe dynamical parameters

From: Harish Srinivasan (
Date: Mon Jan 02 2017 - 02:27:50 CST

Dear all,

A very happy new year.
I am trying to simulate soft matter systems like micelles, vesicles etc. We
calculate various dynamical parameters like diffussivity, velocity
autocorrelation, incoherent inelastic scattering function etc. It is quite
impossible to set up an NVE simulation as it becomes very difficult to
maintain the temperature of the system.

One can use NPT or NVT ensemble in such scenarios. But, there is an
important point of concern about the right choice of friction parameters.
It is well known that the friction parameter can indirectly affect the
values of the dynamical quantities observed and that is why NVE is the best
choice when probing dynamics of the system. So is it appropriate to choose
a friction coefficient (although they might be large) such that the
dynamical parameters match with experimental values ? A plenty of recent
publications have followed this kind of a scheme. But some old publications
also suggest that one should choose this parameter as small as possible so
as to neglect its effects on dynamics.

For example, the diffusivity of TIP3P water
D (expt) = 2.6 x 10^{-5} cm^2/s
D (NVE) = 5.3 x 10^{-5} cm^2/s
D (NVT) = 5.1 x 10^{-5} cm^2/s (friction = 1 /ps)
D (NVT) = 2.9 x 10^{-5} cm^2/s (friction = 5 /ps)

Now, the friction = 5/ps matches well with expt. but not with its NVE
values. So what is the right choice friction parameter here for a truthful
simulation ?

Thank you for your kind inputs and suggestions,

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