Gyration tensor

The gyration tensor is a tensor that describes the second moments of position of a collection of particles

$S_{mn}\equiv {\frac {1}{N+1}}\sum _{i=1}^{N}r_{m}^{(i)}r_{n}^{(i)}$

where $r_{m}^{(i)}$ is the $\mathrm {m^{th}}$ Cartesian component of the position $\mathbf {r} ^{(i)}$ of the $\mathrm {i^{th}}$ particle and which has been defined such that

$\sum _{i=1}^{N}\mathbf {r} ^{(i)}=0$

In the continuum limit,

$S_{mn}\equiv \int d\mathbf {r} \ \rho (\mathbf {r} )\ r_{m}r_{n}$

where $\rho (\mathbf {r} )$ represents the number density of particles at position $\mathbf {r}$.

The gyration tensor is related to the moment of inertia tensor. The chief difference is that the particle positions are weighted by mass in the inertia tensor.

Diagonalization

Since the gyration tensor is a symmetric 3x3 matrix, a Cartesian coordinate system can be found in which it is diagonal

$\mathbf {S} ={\begin{bmatrix}\lambda _{x}^{2}&0&0\\0&\lambda _{x}^{2}&0\\0&0&\lambda _{z}^{2}\end{bmatrix}}$

where the axes are chosen such that the diagonal elements are ordered $\lambda _{x}^{2}\leq \lambda _{y}^{2}\leq \lambda _{z}^{2}$. These diagonal elements are called the principal moments of the gyration tensor.

Shape descriptors

The principal moments can be combined to give several parameters that describe the distribution of particles. The squared radius of gyration is the sum of the principal moments

$R_{g}^{2}=\lambda _{x}^{2}+\lambda _{y}^{2}+\lambda _{z}^{2}$

The asphericity $b$ is defined by

$b\equiv \lambda _{z}^{2}-{\frac {1}{2}}\left(\lambda _{x}^{2}+\lambda _{y}^{2}\right)$

which is always non-negative and zero only for a spherically symmetric distribution of particles. Similarly, the acylindricity $c$ is defined by

$c\equiv \lambda _{y}^{2}-\lambda _{x}^{2}$

which is always non-negative and zero only for a cylindrically symmetric distribution of particles. Finally, the relative shape anisotropy $\kappa ^{2}$ is defined

$\kappa ^{2}\equiv {\frac {b^{2}+(3/4)c^{2}}{R_{g}^{4}}}$

which is bounded between zero and one.

Reference

Mattice WL and Suter UW. (1994) Conformational Theory of Large Molecules, Wiley Interscience. ISBN 0471843385

Theodorou DN and Suter UW. (1985) Macromolecules, 18, 1206-1214.