Hitherto, recent publications that broke the wall of putative chemical understanding and presented detection/isolation of novel compounds with intriguing bonding characters can still be provocative at times. The stir in such discoveries arose partly from the lack of a universally accepted bond descriptor. While bond dissociation energies (BDE) and rigid force constants have been generally regarded as primary tools for such interpretation, they are prone to flawed definition of chemical bonds in certain scenarios whether simple or controversial.
Such reasons prompted the necessity to seek an alternative approach to describe covalent and non-covalent interactions more rigorously. , a German chemist at the TU Braunschweig and his Ph.D. student at the time, Kai Brandhorst, developed a program COMPLIANCE (freely available to the public), which harnesses compliance constants for tackling the aforementioned tasks. The authors use an inverted matrix of force constants, i.e., inverted Hessian matrix, originally introduced by W. T. Taylor and K. S. Pitzer. The insight in choosing the inverted matrix is from the realization that not all elements in the Hessian matrix are necessary—and thus redundant—for describing covalent and non-covalent interactions. Such redundancy is common for many molecules, and more importantly, it ushers in the dependence of the elements of the Hessian matrix on the choice of coordinate system. Therefore, the author claimed that force constants albeit more widely used are not an appropriate bond descriptor whereas non-redundant and coordinate system-independent compliance constants are.