In physics, the gyration tensor is a tensor that describes the second moments of position of a collection of particles
Contents
where
i.e. in the system of the center of mass
Another definition, which is mathematically identical but gives an alternative calculation method, is:
Therefore, the x-y component of the gyration tensor for particles in Cartesian coordinates would be:
In the continuum limit,
where
Although they have different units, the gyration tensor is related to the moment of inertia tensor. The key difference is that the particle positions are weighted by mass in the inertia tensor, whereas the gyration tensor depends only on the particle positions; mass plays no role in defining the gyration tensor.
Diagonalization
Since the gyration tensor is a symmetric 3x3 matrix, a Cartesian coordinate system can be found in which it is diagonal
where the axes are chosen such that the diagonal elements are ordered
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
The asphericity
which is always non-negative and zero only when the three principal moments are equal, λx = λy = λz. This zero condition is met when the distribution of particles is spherically symmetric (hence the name asphericity) but also whenever the particle distribution is symmetric with respect to the three coordinate axes, e.g., when the particles are distributed uniformly on a cube, tetrahedron or other Platonic solid.
Similarly, the acylindricity
which is always non-negative and zero only when the two principal moments are equal, λx = λy. This zero condition is met when the distribution of particles is cylindrically symmetric (hence the name, acylindricity), but also whenever the particle distribution is symmetric with respect to the two coordinate axes, e.g., when the particles are distributed uniformly on a regular prism.
Finally, the relative shape anisotropy
which is bounded between zero and one.