A project funded by The French National Research Agency for the period 01/04/2018 - 31/03/2022.
The main objective of the project is to investigate the effect of quantum electrodynamics on molecular properties.
During the past thirty years it has been realized that relativistic effects have a profound impact on chemistry. It may be asked whether relativity was the last train from physics to chemistry, or whether further refinement is needed by taking into account the effects of quantum electrodynamics (QED). The present project aims at providing a definite answer to this question as well as providing tools for its exploration.
Previous studies indicate that the effect of QED on valence properties, such as electron affinities and ionization energies, is to reduce the relativistic effects by about one percent, which is rather modest. The situation for properties that depend on the electron density in the vicinity of nuclei is less clear. We therefore plan to investigate QED effects in chemistry with emphasis on such properties, notably on NMR parameters, on core and Mössbauer spectroscopies and on electric field gradients (which allows the determination of nuclear electric quadrupole moments).
Another domain in which QED effects may come into play concerns spectroscopic tests of fundamental physics. Spectroscopic experiments of extreme precision have been carried out on atoms and molecules in order to probe the standard model of the universe, as well as alternative models. Examples of such experiments concern the non-conservation of parity of chiral molecules as well as the search for a possible electric dipole moment of fundamental particles such as the electron. These experiments depend on theory for guidance and for the extraction of the quantities of interest. Ultimately the combination of theory with atomic and molecular spectroscopy may allow the determination of physical observables normally obtained from high-energy experiments such as the large hadron collider, but this would require not only experiments, but also theoretical calculations of very high accuracy, hence the need to know the importance of QED effects for such properties.
We address our objective in two steps. In a first pragmatic step we will incorporate effective QED potentials, already available for atoms, into molecular calculations. Our platform for development will be the DIRAC program which is presently the leading program for 2- and 4-component relativistic molecular calculations with extensive functionality for molecular properties. In a second, more ambitious and therefore more risk-prone step we plan to formulate and implement a variational approach to QED in the framework of existing quantum chemical methods such as Hartree-Fock and Density Functional Theory. A key challenge will be real-space renormalization to curb the singularities known from previous QED work, but now to be carried out in the framework of finite basis set expansions. The success of this step hinges on the multidisciplinary character of the molQED team, involving theoretical chemists, physicists and mathematicians.