A New Application of EDSAC to Crystal Structure Analysis

Abstract

IT is well known that the structure of a centrosymmetric crystal could be solved by selecting a number N of structure factors (F(h)'s), and evaluating the electron density for each of the 2^N possible sign combinations on the.F(h)'s. The correct choice of signs would give a recognizably correct structure when N was sufficiently large. For crystals of moderate complexity, experience has shown that the structure can be determined after the correct signs of about twenty of thelargest structure factors have been found. Even when a very rapid means of evaluating a multi-dimensional Fourier series, such as Pepinsky's X.R.A.C.¹, is available, it is not feasible to evaluate and inspect about 2²⁰ (more than a million) electron-density maps. Our application of the EDSAC consists effectively in making the machine survey this large number of alternative answers, and select from them a few which fulfil certain criteria. The following is a brief outline of the method.