We have applied the method of simulated annealing to the refinement of the 3 Å resolution
crystal structure of the influenza virus hemagglutinin glycoprotein, using the program X-
PLOR. Two different methods were introduced into X-PLOR to treat the noncrystallographic
symmetry present in this and in other crystal structures. In the first, only the unique protomer
atoms are refined; by application of the non-crystallographic symmetry operators to the
protomer atoms, the X-ray structure factor derivatives are effectively averaged, and a non …