  Evolutionary crystal structure prediction proved to be a powerful approach
for studying a wide range of materials. Here, we present a specifically
designed algorithm for the prediction of the structure of complex crystals
consisting of well-defined molecular units. The main feature of this new
approach is that each unit is treated as a whole body, which drastically
reduces the search space and improves the efficiency, but necessitates the
introduction of new variation operators described here. To increase diversity
of the population of structures, the initial population and
part($\scriptsize{\sim}$20%) of the new generations are generated using space
group symmetry combined with random cell parameters and random positions and
orientations of molecular units. We illustrate the efficiency and reliability
of this approach by number of tests (ice, ammonia, carbon dioxide, methane,
benzene, glycine and butane-1,4-diammonium dibromide). This approach easily
predicts the crystal structure of methane \emph{A} containing 21 methane
molecules (105 atoms) per unit cell. We demonstrate that this new approach has
also a high potential for the study of complex inorganic crystals on the
examples of a complex hydrogen storage material Mg(BH$_4$)$_2$ and elemental
boron.
