BLOC Instruction
BLOC n1 n2 atomnames
If n1 or n2 are positive, the x, y and z parameters of the named atoms are
refined in the corresponding cycle. If n1 or n2 are negative, the occupation
and displacement parameters are refined in cycle. Not more than two such
parameters may be specified on a single BLOC instruction, but the same atoms
may be mentioned in any number of BLOC instructions. To refine both x, y and
z as well as displacement parameters for an atom in the same block, n1 and n2
should specify the same cycle number, but with opposite signs. A BLOC
instruction with no atom names refines all atoms in the specified cycles.
The pattern of blocks is repeated after the maximum block number has been
reached if the number of L.S. refinement cycles is larger than the maximum
BLOC |n1| or |n2|. If a cycle number less than the maximum |n1| or |n2| is
not mentioned in any BLOC instruction, it is treated as full-matrix. The
overall scale, batch/twin scale factors, extinction coefficient, SWAT g
parameter and free variables (if present) are refined in every block. Riding
(hydrogen) atoms and atoms in rigid groups are included in the same blocks as
the atoms on which they ride.
For example, a polypeptide consisting of 30 residues (residue numbers 1..30
set by RESI instructions) could be refined efficiently as follows (all
non-hydrogen atoms assumed anisotropic):
BLOC 1
BLOC -2 N_1 > O_16
BLOC -3 N_14 > O_30
which would ensure 3 roughly equally sized blocks of about 800 parameters each
and some overlap between the two anisotropic blocks to avoid problems where
they join. The geometric parameters would refine in cycles 1,4,7 .. and the
anisotropic displacement parameters in the remaining cycles. An alternative
good blocking strategy would be to divide the structure into three overlapping
blocks of xyz and Uij parameters, and to add a fourth cycle in which all xyz
but no Uij values are refined (these four blocks would then also each contain
about 800 parameters), i.e.:
BLOC 1 -1 N_1 > O_11
BLOC 2 -2 N_10 > O_21
BLOC 3 -3 N_20 > O_30
BLOC 4