This data block defines constraints on changes to the cell during relaxation or molecular dynamics. It has the following format:
%BLOCK CELL_CONSTRAINTS Ia Ib Ic Iα Iβ Iγ %ENDBLOCK CELL_CONSTRAINTS
The first three entries relate to the magnitude of the three lattice parameters a, b and c and the second set of three entries to the angles α, β and γ.
If the value of the entry corresponding to a magnitude or angle is zero, this quantity will remain fixed. If two or three entries contain the same integer, the corresponding quantities will be constrained to have the same value. If a positive integer greater than 0 occurs in entries 1 through 3 the same integer cannot occur in entries 4 through 6 as this would imply that a vector length and angle must have the same value.
To allow independent optimization of the lattice parameters a, b and c while keeping all three angles fixed, specify:
%BLOCK CELL_CONSTRAINTS 1 2 3 0 0 0 %ENDBLOCK CELL_CONSTRAINTS
CASTEP keyword glossary
CASTEP cell keywords and data blocks