Personal Impressions from WATOC 2020 – Dispersion and non Born-Oppenheimer models.

WATOC 2020 was just held in 2022 in Vancouver Canada, over one week. With many lectures held in parallel, it is not possible for one person to cover anything like the topics presented, so this is a personal view of some of those talks that I attended. As happens with many such events, common themes gradually emerge and here I highlight just two that struck me as important for the future of computational chemistry.

1. Dispersion. This goes back to Fritz London and his formula: E_disp = -(C6/R⁶), where where coefficient C6 depends on the expectation values of the instantaneous dipole moments and average atomic excitation energies. The nature of this formula suggests that it decays rapidly with the distance between any pair of nuclei, R. But an increasing body of evidence is suggesting that such simple approaches (implemented as a correction in many e.g. DFT methods and known as e.g. D3+BJ, or D4 etc) may be underestimating the long range dispersion attractions. One nice example is what is known as the exfoliation of layers of graphite, where the forces holding the layers together can be measured quite accurately and which emerge as a great deal greater than the simple formulae suggest. It appears we now have a renaissance in developing new more accurate dispersion energy methods which include various higher order terms and are being applied to a variety of discrete molecule and solid state systems. One space to look out for!
2. .Non Born-Oppenheimer behaviour. It is a mainstay of most solutions of the Schroedinger equation where the nuclei are treated as classical point charge objects with fixed positions in an electronic field described by a wavefunction. But there is now considerable activity in developing methods that generate an extended Hessian (2nd derivative matrix) describing the forces that depends on both the classical nuclear coordinates of non-hydrogen atoms and the expectation values of quantum proton coordinates. This matrix is diagonalised to obtain the coupled vibrational frequencies which now naturally include the anharmonicity of the now quantum-treated protons and recovers the electron-proton correlation. It impacts most directly on so-called proton tunnelling and isotope effects, which can slice off 2-4 kcal/mol from barriers, but is now seen as a manifestation of electron-proton correlations in non-Born Oppenheimer potentials. The classical approach is to shave these energies off using eg Eckart potentials, but is now being replaced by e.g. a nuclear-electronic orbital method (NEO) which calculate the barriers from first principles. Typical types of reactions that are affected by non-BO behaviour are proton coupled electron transfers (PCET, see here for an example) which are increasingly seen as important in many biological processes.

I have tried to highlight just two themes that emerged from WATOC of personal interest to me; of course there was a great deal of new and exciting stuff that I have not mentioned. The next WATOC will be in Oslo in 2025, and no doubt new and exciting themes will emerge there as well!