  The mean volume reflection angle of a high-energy charged particle passing
through a bent crystal is expressed as an integral involving the effective
interplanar potential over a single crystal period. Implications for positively
and negatively charged particles, and silicon crystal orientations (110) and
(111) are discussed. A generic next-to-leading-order expansion in the ratio
$E/R$ of the particle energy $E$ to the crystal bending radius $R$ is given.
For positively charged particles, the dependence of the mean volume reflection
angle on $E/R$ proves to be approximately linear, whereas for negatively
charged particles the linear behaviour is modified by an $E/R$-dependent
logarithmic factor. Up-to-date experimental data are confronted with
predictions based on commonly used atomic potentials.
