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The m-matrix serves to fix position and orientation of your molecule
during geometry optimizations. It cannot be used to fix internal
coordinates! The m-matrix is a diagonal matrix of dimension
(where is the number of atoms). Normally m will be initialized as
a unit matrix by `relax`. As an example consider you want to restrict
an atom to the xy-plane. You then set the m(z)-matrix element for
this atom to zero. You can use at most six zero m-matrix diagonals
(for linear molecules only five)--corresponding to translational and
rotational degrees of freedom. Note that the condition of the
-matrix can get worse if positional restrictions are
applied to the m-matrix. m-matrix elements violating the molecular
point group symmetry will be reset to one. Non-default settings for
m-matrix diagonals of selected atoms have to be specified within data
group `$m-matrix` as:
$m-matrix
1 0.0 0.0 0.0
10 1.0 0.0 0.0
11 1.0 1.0 0.0

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