Imperial College London

Henry Rzepa

The triennial conference is this year located in
[Munich](http://www.watoc2017.com). With 1500 participants and six
parallel sessions, this report can give only a flavour of proceedings.

1.  Edward Valeev talked about the scaling problem in coupled cluster
    theories, the so-called gold standard for computing the energy and
    properties of small molecules. The problem is that the number of
    basis functions N describing the atomic basis set for the atoms
    scales from between N^6^ to N^10 ^in terms of computer time, with
    similar behaviour for the memory required for the calculation. He
    described methods based on natural pair orbitals and localisation
    schemes which can achieve linear scaling, ie N^1^ for the
    energy, quite a break through! Using reasonable basis sets,
    CCSD(T)-like energies for molecules with 100s of atoms were
    reported. During the Q&A time afterwards (the tight schedules
    associated with so many speakers means questions are often limited
    to 1-2, with very short answers) a question was posed about the
    prospects for first and second derivatives for the method. This
    means that *e.g.* reaction mechanisms can then be probed with
    unprecedented energetic accuracy. The answer was non-committal, but
    if these derivatives do arrive, it will revolutionise our ability to
    understand mechanisms.
2.  Which brings me nicely to Jeremy Harvey, who talked about
    calculating accurate overall rate constants for complex mechanistic
    cycles. The rate equations are solved for the steady state condition
    and include concentrations of all species and the energies are
    obtained using CCSD(T)-F12 theory (a modification which allows
    better basis set scaling without increased computation time) as
    single point geometries. He described an example where the barrier
    associated with a postulated mechanism was about 6 kcal/mol higher
    than derived from the observed rate. This was sufficient to induce
    them to explore alternative mechanisms, which were indeed located
    with an appropriately lower barrier. I
    have[ used](http://www.ch.imperial.ac.uk/rzepa/blog/?p=16902) the
    value of \~10 kcal/mol as my mechanistic test on this blog, and it's
    really nice to see this value being reduced further.
3.  Yet again this theme emerged with Yitzhak Apeloig, who asked about
    the mechanism for C=Si bond rotations in substituted systems
    recently made in his group. The energy of this rotation is low
    enough to be observed in NMR spectra. But when the energy of C=Si
    bond rotation is computed it comes out about 10 kcal/mol too high.
    Again alternative mechanisms were explored and it turns out that a
    1,2 migration from R~2~C=SiR~2~ to form a carbylidene species,
    R-C-SiR~3~, rotation and then 1,2 again to reformulate the
    R~2~C=SiR~2~ system came up with the goods.
4.  Peter Scheiner talked about how attractions between molecules can be
    induced by dispersion. He described how Ph~3~C-CPh~3~ is an unknown
    molecule (dissociating into Ph~3~P^•^ radicals) but when
    4,6-di-tert-butyl groups are placed on all the phenyl rings, the
    dispersion attractions between them can account for \~60 kcal/mol
    (!), more than enough to stabilise the system. I have already
    described some of this work in a [post
    here](http://www.ch.imperial.ac.uk/rzepa/blog/?p=18582). The
    prospects are very exciting for more dispersion-stabilised molecules
    to emerge. During Q&A, a question was posed about what other atom
    pairs other than H...H might be brought into ultra-short contact by
    these attractive dispersion forces; we may expect further examples
    to emerge in the near future.
5.  Ken Houk gave a fascinating glimpse into the post-transition state
    world of reaction dynamics, as applied to Diels Alder cycloadditions
    and Cope rearrangements. The reactions are characterised by the
    residency times of the dynamic trajectories in the region of the
    transition state as short (\~4 fs), medium (20-40fs) and long
    (80+fs), these times mapping on to what we used to call
    "synchronous", "asynchronous" and "stepwise". A good example is the
    so-called [bis-pericyclic reaction of
    cyclopentadiene](http://www.ch.imperial.ac.uk/rzepa/blog/?p=7389)
    where the trajectories pass through a transition state but then
    bifurcate into two (in this case) equivalent pathways. He discussed
    other examples where the trajectories follow either a 2+4
    cycloaddition pathway or a 4+6 alternative pathway and how the
    number of trajectories for each can be influenced by either solvent
    (water) or an enzyme. Ken described several 20-40fs trajectories as
    corresponding to "dynamic stepwise" reactions, which during Q&A was
    suggested are equivalent to the term "hidden intermediate" pathways
    coined by Dieter Cremer and as revealed in many posts here from the
    intrinsic reaction coordinates or IRCs. This is a clear growth area
    and expect many more examples of reaction dynamics to be applied to
    many exciting systems in the future.
6.  Leo Radom talked about very simple molecules, H~3~CX and the effects
    on the bond dissociation energy (BDE) of the C-H bonds if the group
    X is either strongly or weakly protonated (the latter via a hydrogen
    bond), or deprotonated (again strongly or weakly via a hydrogen bond
    from hydroxide anion). This is important in several enzymic
    pathways, where the CH bond might be activated in a similar manner
    by the enzyme. He also talked about similar effects on the
    ionisation potential. I noticed a connection between this theme and
    what might be called the electron affinity of H~3~CX. If you want to
    see what the connection is, go visit the [Aachen bond
    Slam](http://www.ch.imperial.ac.uk/rzepa/blog/?p=18705), about which
    I have previously blogged! 

I will stop with an observation that all the notes above were taken in
real-time during the talks, which all emerged as Powerpoint slides,
having an average residency time on the screen of perhaps 1-2 minutes
each. References were invariably given as full journal citations
(authors, journal, year, volume, pages) rather than as DOIs, and given
the time constraints I did not try to capture them. Hence the lack of
citations above to the presenters' work. The slide displays are
traditionally not made available to audiences^‡^ and photography of the
screen or recording is considered very bad form. Conferences are not
really about FAIR data, which I have described often on this blog.

I hope these six examples give one flavour of what is happening at WATOC
2017. If another interesting collection emerges, I may describe it here.

------------------------------------------------------------------------

^‡^But see e.g. doi: [b9r9](https://doi.org/b9r9) for an Aachen talk.

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The triennial conference is this year located in Munich. With 1500 participants and six parallel sessions, this report can give only a flavour of proceedings. Edward Valeev talked about the scaling problem in coupled cluster theories, the so-called gold standard for computing the energy and properties of small molecules.

Messaggi di Rogue Scholar

WATOC 2017 report.