Locating heavy atoms by integrating direct methods and SIR techniques

Direct methods have been applied to the SIR (single isomorphous replacement) case to estimate structure-factor moduli from diffraction magnitudes. The joint probability distribution function P(E_H,E_p,E_d) has been calculated by explicitly considering, as an additional primitive random variable, the cumulative error arising from measurements and from lack of isomorphism. The specific feature of the approach is that it provides estimated values of |E_H| which depend on the experimental diffraction data as well as on errors. Some test structures have been used to check the efficiency of the new estimates. Patterson techniques, using the estimated |E_H|² values as coefficients, as well as a tangent procedure, have been implemented into a computer program to locate the heavy atoms automatically. All the experimental tests show that the new approach provides results highly competitive with the traditional |E_H|² estimates.