  We present the results of atomistic and ab initio simulation of several
different tilt grain boundaries (GB) in silicon. The boundary structures
obtained with genetic algorithm turned out to have no coordination defects,
i.e. all silicon atoms restored their tetrahedral coordination during the
structure optimisation. That concerns previously known symmetric {\Sigma} 5
(130), {\Sigma} 3 (211) and {\Sigma} 29 (520) boundaries as well as previously
unknown asymmetric {\Sigma} 9 (-255)/(-211), {\Sigma} 3 (-255)/(211) and
{\Sigma} 13 (790)/(3 11 0) structures. We have performed an extensive study of
defect segregation on the boundaries, including neutral vacancy and carbon,
phosphorus and boron impurities. A clear correlation between the segregation
energy of the defect and local geometry of the boundary site where the defect
is segregated has been revealed. We suggest a simple purely geometric model for
evaluation of approximate segregation energies of the listed defects.
