Z-matrix

The Z-matrix is a way to represent a system built of atoms. It provides a description of each atom in a molecule in terms of its atomic number, bond length, bond angle, and dihedral angle, the so-called internal coordinates. Z-matrices can be converted to cartesian coordinates and back, the information content is identical. It is used for inputting the geometries of simple molecules in many molecular modelling and computational chemistry programs.

Because Z-matrices contain molecular connectivity information, certain quantum chemical calculations, such as geometry optimization, may be performed faster, because an educated guess is available for an initial Hessian matrix, and more natural internal coordinates are used rather than Catesians.