Some B_eq are more equivalent than others

Crystallographic structural models for macromolecules have typically included an isotropic displacement parameter B_iso for each atom. In cases where the structural model instead includes anisotropic displacement parameters U^ij, the derived quantity B_eq can be substituted for B_iso for many purposes. B_eq is not, however, the best predictor of the value B_iso that would hypothetically have been obtained by direct refinement of an isotropic model. A new entity B_est is proposed that represents an estimate for B_iso that minimizes the Kullback-Leibler divergence from a paired anisotropic model. In general B_est/B_eq < 1, with the difference between the two values becoming larger for atoms that are more anisotropic. Although this difference does not affect direct refinement of either isotropic or anisotropic models, it is relevant to any analysis that compares isotropic and anisotropic models of the same underlying structure. In particular, it may lead to improved selection of multi-group TLS models based on analysis of an initial isotropic refinement.