The m-Matrix

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 3n² (where n 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 BmB^†-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