Geometry Optimisation with CASTEP

2 O j
2 O j
2 O j
%ENDBLOCK IONIC_CONSTRAINTS
For two constraints, one involving two sulfur ions, and another involving three oxygen ions. All
coefficients not specified are assumed to be zero.
The first column in the block gives a unique number to the constraint specified. The second specifies the
species by either atomic symbol (S or O in the above example) or atomic number. The third column is
the index within a species
current constraint are then specified.
. The co-efficients of the three spatial co-ordinates for this ion under the
As an example, let us consider the case of restricting a single ion to move along along a plane parallel to
. The normal to this plane is and our constraint is that the dot product of this
normal with the position vector of the ion in question is zero. If this is the second ion in the fourth
species then
In this trivial example, we can see that we need (if the fourth species is say, sulfur)
%BLOCK IONIC_CONSTRAINTS
1 S 2 -1 1 0
%ENDBLOCK IONIC_CONSTRAINTS
to satisfy the above equation.
A variety of constraints can be specified in this fashion. For simplicity, a special cell file keyword is
provided to fix the centre of mass in the calculation.
fix_com = true