This data block defines constrained internal coordinates (bonds, angles, torsions). These internals are held fixed at the values of the initial structure.
The data block has the following format:
%BLOCK NONLINEAR_CONSTRAINTS CONSTRAIN_TYPE atom1 atom2 (atom3 atom4) CONSTRAIN_TYPE atom1 atom2 (atom3 atom4) . . . %ENDBLOCK NONLINEAR_CONSTRAINTS
The first element on each line is a constraint type (distance, bend or torsion). Then depending on the type of the constraint two, three or for fields describing atoms involved in the constraint appears. Each field has the following format:
CCC I N1 N2 N3
The first entry is the symbol species of the ion to which this constraint applies. The second element is the number of the ion within the species. The final three numbers are integer numbers specifying the periodic image where the atom is located.
The ordering of the ions in a species is the order in which they appear in the POSITIONS_FRAC or POSITIONS_ABS block in the cell definition file.
%BLOCK NONLINEAR_CONSTRAINTS distance H 4 0 0 0 O 2 0 1 0 bend H 5 0 0 0 C 1 1 0 1 H 2 0 0 0 torsion H 6 0 0 0 H 3 1 0 0 H 1 0 0 1 H 9 1 1 0 %ENDBLOCK NONLINEAR_CONSTRAINTS
This input fixes:
CASTEP keyword glossary
CASTEP cell keywords and data blocks