Structure-factor extrapolation using the scalar approximation: theory, applications and limitations

The scalar approximation to structure-factor extrapolation (SASFE) procedure is used to extrapolate structure-factor amplitudes for a fully `activated' crystal from amplitudes of a native and a partially activated crystal. Here, it is investigated whether and under what conditions the approximations inherent in SASFE introduce significant errors. The results indicate that the errors introduced by SASFE's assumptions are negligible and that SASFE's usefulness in macromolecular crystallography is limited primarily by the propagation of experimental uncertainties.