Given by \[\Delta \theta _{i- jkl}=\frac{\Delta z_{j}}{r_{eij}}\ \sin \mathit{\Phi }_{kil}\] where the numbering of the atoms is given in the diagram. Φ k i l denotes the angle between the bonds ik and il, Δ z j the perpendicular distance of the atom j from the instantaneous plane ikl and r e i j the equilibrium length of the bond ij.