Figure 1

Geometrical factors that determine (a) triple and (b) one out of three quadruple interactions of three atoms: i, k, p. C is the center of the shortest (i-p) side of triangle; γ = γ _pi ; ${\overline{\omega }}_{\Delta }=\left({\omega }_i+{\omega }_k+{\omega }_p\right)/3$; ${\overline{\omega }}_{\iff }=\text{[}2{\omega }_i+{\omega }_k+{\omega }_p+{\omega }_k{\omega }_p/2{\omega }_i+{\omega }_i^2/({\omega }_k+{\omega }_p)]/5$.