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The Debye–Waller factor (DWF), named after Peter Debye and Ivar Waller, is used in condensed matter physics to describe the attenuation of x-ray scattering or coherent neutron scattering caused by thermal motion. It has also been called the B factor or the temperature factor. Often, "Debye-Waller factor" is used as a generic term that comprises the Lamb-Mössbauer factor of incoherent neutron scattering and Mössbauer spectroscopy.

The DWF depends on the scattering vector q. For a given q, DWF(q) gives the fraction of elastic scattering; 1 - DWF(q) correspondingly gives the fraction of inelastic scattering. (Strictly speaking, this probability interpretation is not true in general.) In diffraction studies, only the elastic scattering is useful; in crystals, it gives rise to distinct Bragg peaks. Inelastic scattering events are undesirable as they cause a diffuse background — unless the energies of scattered particles are analysed, in which case they carry valuable information (inelastic neutron scattering).

The basic expression for the DWF is given by

$\text{DWF}={⟨\exp (i\mathbf{q}\cdot \mathbf{u})⟩}^2$

where u is the displacement of a scattering center, and <...> denotes either thermal or time averaging.

Assuming harmonicity of the scattering centers in the material under study, the Boltzmann distribution implies that $\mathbf{q}\cdot \mathbf{u}$ is normally distributed with zero mean. Then, using for example the expression of the corresponding characteristic function, the DWF takes the form

$\text{DWF}=\exp (-⟨[\mathbf{q}\cdot \mathbf{u}{]}^2⟩)$

Note that although the above reasoning is classical, the same holds also quantum mechanically.

Assuming also isotropy of the harmonic potential, one may write

$\text{DWF}=\exp (-q^2⟨u^2⟩/3)$

where q, u are the magnitudes (or absolute values) of the vectors q, u respectively, and $⟨u^2⟩$ is the mean squared displacement. Note that if the incident wave has wavelength $\lambda$, and it is elastically scattered by an angle of $2\theta$, then

$q=\frac{4\pi \sin (\theta )}{\lambda }$

In the context of protein structures, the term B-factor is used. The B-factor is defined as:

$B=8{\pi }^2⟨u^2⟩$

It is measured in units of Ų. The B-factors can be taken as indicating the relative vibrational motion of different parts of the structure. Atoms with low B-factors belong to a part of the structure that is well-ordered. Atoms with large B-factors generally belong to part of the structure that is very flexible. Each ATOM record (PDB file format) of a crystal structure deposited with the Protein Data Bank contains a B-factor for that atom.

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