From Eq. (10.71) in the book “Thermodynamics of Crystal”, atomic
displacement, **u**, is written by

where *j* and *l* are the labels for the *j*-th atomic position in the
*l*-th unit cell, *t* is the time, is an axis (a
Cartesian axis in the default behavior of phonopy), *m* is the atomic
mass, *N* is the number of the unit cells, is the
wave vector, is the index of phonon mode. *e* is the
polarization vector of the atom *jl* and the band at
. is the atomic position and
is the phonon frequency. and
are the creation and annihilation operators of
phonon. The expectation value of the squared atomic displacement is
calculated as,

where is the phonon population, which is give by,

where *T* is the temperature, and is the
Boltzmann constant. The equation is calculated using the commutation
relation of the creation and annihilation operators and the
expectation values of the combination of the operations, e.g.,

Mean square displacement matrix is defined as follows:

This is a symmetry matrix and diagonal elements are same as mean square displacement calculated along Cartesian x, y, z directions.

In phonopy, eigenvectors are calculated in the Cartesian axes that are defined in the input structure file. Mean square displacement along an arbitrary axis is obtained projecting eigenvectors in the Cartesian axes as follows:

where is an arbitrary unit direction.

According to the paper by Grosse-Kunstleve and Adams [J. Appl. Cryst.,
35, 477-480 (2002)], mean square displacement matrix in the cif
definition (`aniso_U`

),
, is obtained by

where is the matrix to transform a point in fractional coordinates to the Cartesian coordinates and is the diagonal matrix made of reciprocal basis vector lengths as follows:

and

, , are defined without .