Experience with fast Fourier least squares in the refinement of the crystal structure of rhombohedral 2-zinc insulin at 1.5 Å resolution

The newly developed fast Fourier least-squares algorithm [Agarwal (1978). Acta Cryst. A34, 791-809] has been used to refine the structure of rhombohedral 2-Zn pig insulin at a resolution of 1.5 Å. The CPU time required for each cycle of refinement was about 3 min on an IBM 370/168 computer. After 67 cycles of refinement the conventional R factor was 0.113 for 11 890 terms. The procedure used in the refinement is described and an analysis of the results presented. From the experience learned in this refinement a procedure is suggested which should provide faster convergence and require considerably fewer refinement cycles.