A molecular geometry can be described in cartesian coordinates. By using this type of coordinates, the bonds and the angles among atoms are not explicitly defined. Software such as Gaussian09, QChem, and CFOUR can read input geometry expressed in cartesian coordinates. Specific keywords might be required.
Cartesian coordinates are often used in those cases where an 180° (or close to 180°) angle among three atoms is present in the molecular geometry. This is an expedient to avoid the use of one or more ghost /dummy atoms in z-matrix atomic-type coordinates.
Example of cartesian coordinates for the water molecule:
O 0.00000 0.00000 0.00000
H 0.00000 0.40705 -0.23228
H 0.00000 -0.40705 -0.23228