 The Eckart conditions give an orthogonal direct sum decomposition of this space
The elements of the 3"N"-6 dimensional subspace Rint are referred to as "internal coordinates", because they are invariant under overall translation and rotation of the molecule and, thus, depend only on the internal (vibrational) motions.The elements of the 6-dimensional subspace Rext are referred to as "external coordinates", because they are associated with the overall translation and rotation of the molecule.To clarify this nomenclature we define first a basis for Rext.