wavefunctions, ELF and high-resolution X-ray crystallography. The spirit of that
book combined an educational style with an awareness of current scientific
boundaries, while avoiding too many equations in the main text, as requested by the
editors. That chapter managed to deliver added value in explaining QTAIM again,
by means of an alternative angle of exposition, different to that in other sources [3–
7]. Moreover, an historic narrative was given there, as well as a discussion of
topological energy partitioning. The current chapter selects and re-explains elements from that document, with the new didactic example of HCN, justifies the
name of Quantum Chemical Topology (QCT) (which encompasses and supersedes
QTAIM), outlines the current state of affairs in a novel QCT-based protein force
field, and briefly invites the community to start falsifying interpretative methods
(QCT and non-QCT) in case studies where the outcome would make a difference.
The term Quantum Chemical Topology (QCT) was first coined [8] in 2003, and
the first dedicated symposium took place in 2013, in Mexico City. Footnote 19 in
the paper that coined QCT, gave a detailed justification for this name and it is
helpful to quote part of this footnote, with a few modifications: “…The use of the
acronym QCT does not downplay the physics behind “Atoms in Molecules”
(AIM) by referring to the topology language as the central idea behind it. Instead,
the name QCT seeks to capture better what this approach is about. The term (QT)
AIM is widely used but is actually too narrow because, strictly speaking, it only
makes sense as a term if one analyses the electron density topologically. Only then
does one recover an atom in a molecule. A topological analysis of the Laplacian of
the electron density (which is part of AIM) or the topology of the electron localization function (ELF), for example, does not yield atoms in molecules. However,
they can both be put under the umbrella of QCT since they share the central
topological idea. Also, returning to the electron density, one could use the topological analysis to recover molecules inside van der Waals complexes, an important
idea in intermolecular forces. Again, as a name, AIM would not describe this result.
The name QCT also invites any future developments based on a topological
analysis of other 3D or higher-dimensional scalar functions.” This view was
elaborated in Sect. 2.2 of a chapter [9] published in 2005, and updated again in the
introduction of a paper [10] in 2009, and finally in Box 8.1 in Chap. 8 of the
aforementioned book [2] edited by Frenking and Shaik.
The current book should be the right habitat to start thinking more in terms of
falsification when interpreting a chemical phenomenon. Unfortunately, not many
papers directly and critically compare methods. The papers that do so, however,
often terminate with diplomatic and almost vague conclusions. This status quo
perhaps adds to the prevailing notion that all methods are equivalent in quality and
predictive value. They can all be used at the same time, in spite of their known
pitfalls, and even if their results contradict each other. Such view is echoed in
Hoffmann’s statement that “any rigorous definition of a chemical bond is bound to
be impoverishing” and also in his advice “that one should have fun with the fuzzy
richness of the idea”. Such an attitude perpetuates discussions, without prospect of
them ever being resolved. Is this really the fate of interpretative theoretical
chemistry? Or should one strive for conceptual hygiene? Is chemistry really this
24
P.L.A. Popelier
book combined an educational style with an awareness of current scientific
boundaries, while avoiding too many equations in the main text, as requested by the
editors. That chapter managed to deliver added value in explaining QTAIM again,
by means of an alternative angle of exposition, different to that in other sources [3–
7]. Moreover, an historic narrative was given there, as well as a discussion of
topological energy partitioning. The current chapter selects and re-explains elements from that document, with the new didactic example of HCN, justifies the
name of Quantum Chemical Topology (QCT) (which encompasses and supersedes
QTAIM), outlines the current state of affairs in a novel QCT-based protein force
field, and briefly invites the community to start falsifying interpretative methods
(QCT and non-QCT) in case studies where the outcome would make a difference.
The term Quantum Chemical Topology (QCT) was first coined [8] in 2003, and
the first dedicated symposium took place in 2013, in Mexico City. Footnote 19 in
the paper that coined QCT, gave a detailed justification for this name and it is
helpful to quote part of this footnote, with a few modifications: “…The use of the
acronym QCT does not downplay the physics behind “Atoms in Molecules”
(AIM) by referring to the topology language as the central idea behind it. Instead,
the name QCT seeks to capture better what this approach is about. The term (QT)
AIM is widely used but is actually too narrow because, strictly speaking, it only
makes sense as a term if one analyses the electron density topologically. Only then
does one recover an atom in a molecule. A topological analysis of the Laplacian of
the electron density (which is part of AIM) or the topology of the electron localization function (ELF), for example, does not yield atoms in molecules. However,
they can both be put under the umbrella of QCT since they share the central
topological idea. Also, returning to the electron density, one could use the topological analysis to recover molecules inside van der Waals complexes, an important
idea in intermolecular forces. Again, as a name, AIM would not describe this result.
The name QCT also invites any future developments based on a topological
analysis of other 3D or higher-dimensional scalar functions.” This view was
elaborated in Sect. 2.2 of a chapter [9] published in 2005, and updated again in the
introduction of a paper [10] in 2009, and finally in Box 8.1 in Chap. 8 of the
aforementioned book [2] edited by Frenking and Shaik.
The current book should be the right habitat to start thinking more in terms of
falsification when interpreting a chemical phenomenon. Unfortunately, not many
papers directly and critically compare methods. The papers that do so, however,
often terminate with diplomatic and almost vague conclusions. This status quo
perhaps adds to the prevailing notion that all methods are equivalent in quality and
predictive value. They can all be used at the same time, in spite of their known
pitfalls, and even if their results contradict each other. Such view is echoed in
Hoffmann’s statement that “any rigorous definition of a chemical bond is bound to
be impoverishing” and also in his advice “that one should have fun with the fuzzy
richness of the idea”. Such an attitude perpetuates discussions, without prospect of
them ever being resolved. Is this really the fate of interpretative theoretical
chemistry? Or should one strive for conceptual hygiene? Is chemistry really this
24
P.L.A. Popelier