Linking elastic moduli to shape fluctuations I am simulating a polymer network shaped as a sphere. I would like to compute its elastic moduli (bulk and Young's, for example) not by their shear-strain definition but by looking at the fluctuations in volume and shape, which are estimated by using a convex hull construction.
The bulk modulus, being the inverse of the compressibility, can be readily computed by looking at the fluctuations of the volume, viz.:
$$
\beta K = \frac{\langle V \rangle}{\langle V^2 \rangle - \langle V \rangle^2}
$$
As for the Young's modulus, I haven't found any definitive answer on how to compute it in a similar way. The only papers I have found provide relations for 2D systems (for example here). I have tried to naively extend these relations to 3D, obtaining
$$
\beta Y = \frac{1}{\langle V \rangle} \frac{\langle l \rangle^2}{\langle l^2 \rangle - \langle l \rangle^2}
$$
I have no solid basis to support the above relation, and I would like to know whether it is correct and, if it is, how it can be derived.
 A: I am answering my own question as I have found a way of computing the elastic moduli by looking at the equilibrium fluctuations of the shape and volume of a "soft" object. The protocol (adapted from here) is the following:


*

*Generate an ensemble of equilibrium configurations.

*For each equilibrium configuration compute its surface. I have done this by using the convex hull construction.

*Construct a Green-Lagran tensor $\mathbf{C}$ that measures the instantaneous deformation of an object. In order to do this, one has to first choose a stress-free reference configuration

*Compute the tensor invariants that are related to the type of fluctuations you think are relevant. In my case I am interested in the volume and shape, to which we can associate the following invariants:
$$
\begin{align}
J & = \sqrt{\mathrm{det}(\mathbf{C})}\\
I_1 & = \mathrm{tr}(\mathbf{C})J^{-2/3}\\
I_2 & = \frac{1}{2}\left[ \mathrm{tr}^2(\mathbf{C}) - \mathrm{tr}(\mathbf{C}^2)\right]J^{-4/3}
\end{align}
$$

*Write down an expression for the energy of the system due to the thermal excitations of these quantities. Since I am dealing with a polymer network I use the phenomenological Mooney-Rivlin theory for rubber elasticity which can be written as:
$$
U(J, I_1, I_2) = U_0 + V\left(\frac{K}{2}(J - 1)^2 + C_{10}(I_1 - 3) + C_{01}(I_2 - 3)\right)
$$
where $K$ is the bulk modulus and $G = 2(C_{01} + C_{10})$ is the shear modulus.

*If we assume that the deformations are statistically independent, we can fit $K$, $C_{10}$ and $C_{01}$ by looking at the probability distributions of $J$, $I_1$ and $I_2$, as computed over all the equilibrium conformations.


More details can be found here and here.
