DOI:10.1002/chem.200600057 A Model of the Chemical Bond Must Be Rooted in Quantum Mechanics, Provide Insight, and Possess Predictive Power JordiPoater, [a] Miquel Solà,* [b] and F.Matthias Bickelhaupt* [a] Introduction Science is observation and experiment followed by appeal to theory and a return to experiment. This is also the first sentence of the preceding paper, [1] a rebuttal by Bader to an earlier article of ours in the same issue [2] in which we have falsified Bader)s hypothesis that there would be HH bond- ing in planar biphenyl. [3] With this statement we wholeheart- edly agree (how could one disagree). But much of what fol- lows in Bader)s rebuttal [1] is flawed. The present paper is a response in which we address the weaknesses in Bader)s core arguments and statements, and in his theory of Atoms-in-Molecules (AIM). [4] To this end, we briefly review the characteristics of our own, quantitative molecular orbital (MO) model and the associated energy– decomposition approach (EDA) which are firmly rooted in physics. [5–7] This is then contrasted with AIM theory. In the course, we respond to some issues raised by Bader amongst which the He@adamantane inclusion complex [1, 8] and the origin of the higher stability of phenanthrene compared to anthracene. We anticipate here that our bonding analyses and computational experiments falsify the hypotheses, de- rived from AIM, of the presence of He  C bonding in He@ adamantane and HH bonding as the origin of the higher stability of phenanthrene compared to anthracene. It is fair to stress, however, that we are not the first to recognize the flaws in AIM theory. For example, Haaland et al., [8] Cio- slowski et al., [9] Frenking, [10] and, much earlier, Ruedenberg and Feinberg [11] preceded us in raising fundamental concerns about AIM theory. Discussion It is a necessary but, in our view, insufficient requirement for a model of the chemical bond that it is rooted in quan- tum mechanics and that it is able to correctly reproduce all observable quantities, in particular bond energy and molecu- [a] Dr. J. Poater, Dr. F.M. Bickelhaupt Afdeling Theoretische Chemie, Scheikundig Laboratorium der Vrije Universiteit De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands) Fax: (+ 31) 20-598-7629 E-mail: FM.Bickelhaupt@few.vu.nl [b] Prof. Dr. M. Solà Institut de Química Computacional and Departament de Química Universitat de Girona Campus de Montilivi, 17071 Girona, Catalonia (Spain) Fax: (+ 34) 972-418-356 E-mail: miquel.sola@udg.es Abstract: In this response to the pre- ceding paper by Bader, we show that the core arguments and statements pre- sented in the latter are flawed. We argue that it is insufficient for a model of the chemical bond to be rooted in quantum mechanics. A good model must in addition provide insight and possess predictive power. Our molecu- lar orbital (MO) model of the chemical bond, in particular, the associated energy–decomposition approach satis- fies all these conditions. On the other hand, Atoms-in-Molecules (AIM) theory is only rooted in quantum me- chanics as far as its mathematical framework is concerned. The physical status of its central concepts is not so clear. In particular, “bond paths” and “bond critical points” are once more confirmed not to be indicators of a sta- bilizing interaction. Moreover, AIM theory lacks any predictive power. We also address specific questions raised in the preceding paper. Finally, interpret- ing chemical bonding implies choosing a perspective on this phenomenon. That there are many perspectives is a matter of fact and this is in no way un- physical. What is unscientific is to claim uniqueness and truth for one of these choices, namely AIM, and to dis- miss on this ground all other ap- proaches. Keywords: atoms-in-molecules (AIM) theory · bond energy decomposition · bonding theory · density functional calculations · molecular orbital theory · steric repulsion # 2006 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim Chem. Eur. J. 2006, 12, 2902 – 2905 2902