CCL: atomic population analysis



 Sent to CCL by: Peeter Burk [peeter.burk_+_ut.ee]
 Dear Tom,
 
I can understand your passion against atomic charges, and I do believe that your work on them is methodologically as scientific as it can be.
 
But if you compare the atomic charges with airplane, then for tha airplane there is a clear "moment of truth" - will it fly? What is the similar thing for atomic charges? Can you provide experimental charges? How are those MEASURED? To my limited knowledge there are no such experiment, which will measure atomic point charges... If you could convince me that comparison to experiment is possible I would not argue any more, but at the moment the discussion reminds me rather a theology than science as one party does not believe that atomic point charges can be obtained from experiment and other one does...
 Best regards
 Peeter
 On 09/12/2015 04:24 PM, Thomas Manz thomasamanz]-[gmail.com wrote:
 
 Hi Robert,
 The notion of atomic population analysis methods as being arbitrary
 reflects the practical state of affairs in decades past. It is certainly
 true that the earliest methods such as Mulliken and Lowdin populations
 are inherently arbitrary because they lack a basis set limit. But, the
 notion of arbitrariness doesn't accurately characterize the most
 recently developed methods which not only have a well-defined basis set
 limit but also have been developed with extensive and rigorous
 comparisons to experimental data.
 At the time the textbooks you mentioned were written, things had only
 begun to improve in the area of atomic population analysis. I'm sure the
 authors of those textbooks did the best they could with the information
 available at that time. Since those textbooks were written, newer
 methods have been developed that are at least an order of magnitude more
 accurate in comparisons to experiments than the crude, early methods. If
 one were going to write a textbook today, it would be appropriate to say
 that many of the early atomic population analysis methods were arbitrary
 but that some of the most recent ones have been developed through a
 legitimate scientific design process.
 This is an area in which I currently do research. In my research group,
 atomic population analysis methods are developed using scientific
 methods. The procedure we use is not unlike the one used to design
 airplanes. Yes, there is some flexibility in the design of an airplane.
 One could make it longer or shorter, for example. Yet, it is not quite
 accurate to say the design of an airplane is arbitrary. Airplanes, like
 my atomic population analysis methods, are designed to meet certain
 performance criteria. An airplane should fly, for example. Not only
 should it fly, but it should have stable control, take off and land
 smoothly, etc. There is some flexibility when choosing the shape of
 airplane, but it is not quite accurate to say the shape of an airplane
 is arbitrary. Proposed airplane shapes are tested in wind tunnels to see
 how they react to air turbulence, how much drag they produce, etc. There
 is a real engineering design element involved with scientific process of
 engineering and testing prototypes to continuously improve the design.
 Saying that airplane designs are arbitrary somehow doesn't do justice to
 the enormous amount of design work, prototype building, and scientific
 testing that goes into producing an efficient airplane.
 The same principle applies to the development of accurate atomic
 population analysis methods in my research group. We use a legitimate
 and rigorous process that involves engineering design, prototype
 building and scientific testing with comparisons to experimental data. I
 realize that many other research groups do not use such a rigorous
 process, but if you are going to say that atomic population analysis
 methods are arbitrary, please restrict this designation to those that
 actually are arbitrary and mention that some of the recent efforts use a
 legitimate scientific design process.
 The diborane molecule you mentioned does present an interesting example.
 Please find below the net atomic charges and bond orders I computed for
 this molecule:
 B atomic charge: -0.0221
 bridging H atomic charge: 0.131
 outer H atomic charge: -0.054
 B-H(bridging) bond order: 0.423
 B-B bond order: 0.627
 B-H(outer) bond order: 0.940
 sum of bond orders for B atom: 3.39
 sum of bond orders for bridging H atom: 0.91
 sum of bond orders for outer H atom: 1.01
 Sincerely,
 Tom
 On Fri, Sep 11, 2015 at 2:25 PM, Robert Molt
 r.molt.chemical.physics%x%gmail.com <http://gmail.com/>; <owner-chemistry
 . ccl.net <mailto:owner-chemistry . ccl.net>> wrote:
     There is nothing problematic with saying "there is no such thing as
     the quantum mechanical operator for atomic charge." Any atomic
     charge model requires an /arbitrary /partitioning of density as
     "belonging" to certain atoms. None of the laws of physics are
     written in terms of atoms! We don't write the force between atoms,
     we write the force between charges. Trivializing the problem of
     partitioning is brushing under the rug the inherent problem: we
     cannot partition it without arbitrary choices.
     An atomic charge model is especially problematic when the electron
     density is delocalized. There is no way to say to "whom" the
 density
     "belongs" in diborane or a metal conducting a current.
     Moreover, this is the accepted view of the community. See Cramer,
     chapter 9; see Jensen's book (don't recall the chapter; see Szabo
     and Ostlund, chapters 1-3.