Another example is given by the Z-matrix (Table ) of 2-methylpropane. The parameter t denotes a coupled movement of C-C stretches and C-C-C bends with one C-C bond growing exponentially. The electronic structure calculation uses the same parameters as before and with starting values of t = 0 and r CH = 1 Å the calculation converges after 10 iterations to t = -0.054 and r CH = 1.089 Å. It has to be stressed that it would be very complicated to define similar constraints in cartesian coordinates. The parameter t appears both in the bond lengths and angles; subsequent hydrogens in Table depend on the previous carbon atoms. Both facts imply that in cartesian coordinates the constraints would have to appear in the x, y, and z coordinates of nearly every atom and would involve complicated trigonometric expressions, while the same constraints in Zmatrix coordinates (Table ) are user-friendly to define and read.