A VARIANT TO THE RANDOM APPROXIMATION OF THE REFERENCE-FREE ALIGNMENTALGORITHM

Single-particle averaging from electron microscope images strongly dep ends on alignment. Most alignment procedures are based on cross-correl ation of an initial reference image with the rest of the population, l eading to a clear pattern dependence. Among the different approaches t hat have been proposed to minimize this problem, one of the most widel y used is the so-called iterative reference-free alignment algorithm ( RFAA), proposed by Penczek et al. [Ultramicroscopy 40 (1992) 33]. To a void the pattern dependence shown by the initial ''random approximatio n'' step of this method, we propose a variant of the algorithm that is more independent of the input order of the initial images and which c ould substitute the random initialization of the RFAA.