Some notation to be used in this report for basis vectors
and their components is summarized here. The local instantaneous
atomic position vector is denoted by **r**+**u**, with
**r** the mean atomic position vector and **u** the displacement
vector of an atom from its mean position. We use the symbols , and , as is common, for the lengths of the reciprocal axes.

Quantities referred to the direct lattice basis, **a**,
**b**, **c** (or equivalently )
:

Components of **r**: *x, y, z* (or equivalently ).

Components of **u**: (or
equivalently ).

Related anisotropic displacement parameter: ; (dimensionless).

Quantities referred to the basis (or equivalently ):

Components of **r**: (or equivalently ).

Components of **u**: (or
equivalently ).

Related anisotropic displacement parameter:
; (length)^{2} .

Quantities referred to the Cartesian basis ):

Components of **r**: (or
equivalently ).

Components of **u**: (or equivalently ).

Related anisotropic displacement parameter: ;
(length)^{2} .