Calculation of an OMIT map

A space-group-general computer program to compute an OMIT map using fast Fourier transforms is presented. In this procedure an asymmetric unit of the unit cell is divided into several boxes. For each box a set of phases is calculated by the inverse Fourier transforms of the existing electron density distribution which has been modified so that the electron density values in and around that box is a constant. An OMIT map is then calculated for each box by the Fourier transforms of the observed amplitudes with these phases. The computer program may be used to reduce bias in electron density maps during model building and refinement of macromolecules. The procedure was used to detect error in the atomic model of a derivative of ribonuclease (personal communication by J. Nachman, National Cancer Institute, Frederick, Maryland, USA).