Dear colleagues,
One of
my graduate students is in the process of preparing a YouTube video on the topic
of computing bond orders using the method introduced in the following
paper:
T. A. Manz, "Introducing DDEC6 atomic population analysis:
part 3. Comprehensive method to compute bond orders," RSC Advances, 7
(2017) 45552-45581 (open access) DOI: 10.1039/c7ra07400j
The video will
be a less technical presentation, emphasizing chemical concepts with less focus
on mathematics.
Were any of the concepts presented in the above paper
unclear to you? If so, could you please explain which of those concepts you did
not understand? If there is a pattern of some aspects being unclear, then I
would like to know so that it can be explained in a more understandable
way.
If any of you have not taken the time to carefully read that paper,
I believe it would be worth your while to do so. Bond order is a foundational
chemical concept that has wide-ranging impacts and numerous applications
throughout the chemical sciences. The above paper presents the first
comprehensive and computationally efficient method to compute accurate bond
orders across an extremely wide range of material types.
This method to
compute bond orders enabled the first study of quantum-mechanically computed
bond orders for a large number of diatomic molecules:
T. Chen and T. A.
Manz, "Bond orders of the diatomic molecules," RSC Advances, 9 (2019)
17072-17092 (open access) DOI: 10.1039/c9ra00974d
In this work, bond
orders were quantum-mechanically computed for 288 diatomic molecules and ions,
which is >10 times the number of diatomics for which bond orders were
quantum-mechanically computed in each prior work.
Because diatomic
molecules are the smallest molecules containing a chemical bond, they are
natural textbook examples for studying bond order. Therefore, I view the
accurate bond orders for diatomic molecules as foundational to chemical theory.
Even among the diatomic molecules, there are many interesting effects that you
may not be familiar with yet. These can often provide insights that are helpful
to understand larger materials with more atoms.
In two recent papers, the
above bond order method was applied to identify misbonded atoms in the
experimentally-derived crystal structures of metal-organic frameworks:
T.
Chen and T. A. Manz, "Identifying misbonded atoms in the 2019 CoRE
metal–organic framework database," RSC Advances, 10 (2020)
26944-26951 (open access) DOI: 10.1039/d0ra02498h
T. Chen and T.
A. Manz, "A collection of forcefield precursors for metal-organic
frameworks," RSC Advances, 9 (2019) 36492-36507 (open access) DOI:
10.1039/c9ra07327b
For example, carbon atoms in organic compounds often
have a sum of bond orders (SBOs) of approximately 4, because they have four
electrons to share in covalent bonding. Therefore, in the above two studies,
carbon atoms having abnormally low or abnormally high SBOs were flagged as
misbonded. (A low carbon SBO might be caused by a missing hydrogen atom that was
not reported in the crystal structure.) This kind of screening would have been
much harder or perhaps infeasible without the above method to compute bond
orders.
Another important application of these bond orders is to
understand changes that occur during chemical reactions. For example, several
studies reported changes in these bond orders during catalytic
reactions.
Finally, last year an interesting paper explored correlations
between these bond orders and crystal orbital Hamilton populations (a bond
energy projection method):
R.Y. Rohling, I.C. Tranca, E.J.M. Henen, and
E.A. Pidko, "Correlations between density-based bond orders and
orbital-based bond energies for chemical bonding analysis," J. Phys. Chem.
C, 123 (2019) 2843-2854 DOI: 10.1021/acs.jpcc.8b08934
Within the same
material class, the bond order between two specific chemical elements was shown
to be proportional to the COHP. The bond order is easier to interpret than the
COHP.
An encouraging sign is this bond order method is starting to gain
some traction in VASP and CP2K calculations (using the Chargemol code for
post-processing), which going back >3 years bond order calculations using
those codes were nearly unheard of.
I believe the impact could be even
much larger, which is why I'm reaching out to try to highlight some of the
use cases for this method as well as to give you an opportunity to explain to me
what aspects of the method you are having trouble
understanding.
Sincerely,
Tom Manz