Multiscale simulation of magnetic resonance physics

An emerging paradigm in theoretical molecular and materials sciences is computational spectroscopy, which in the case of magnetic resonance spectroscopy means computing directly the primary experimental observables, the time-dependent magnetization or the resulting spectrum. This is to be compared with the traditional way in which comparison with experiment is done at the level of NMR spectral parameters or relaxation times, which themselves result from applying theoretical models to the measurement data. We realise computational spectroscopy by multiscale modelling of

  1. atomic motion within the material via molecular dynamics simulations;
  2. electronic interactions of the spins at each set of atomic positions via advanced quantum-chemical calculation of a time series of spin Hamiltonians, and
  3. the resulting time dependence of the nuclear and electron spin density matrices obtained by integrating the Liouville-von Neumann equation.

This procedure enables detailed, microscopic interpretation and development of relaxation and polarisation transfer processes of relevance to modern magnetic resonance.

Movie 1: Trajectory of an individual Rb-Xe pair in a spin-exchange optical pumping simulation. The bond length (green) of the van der Waals complex and the polarisation (blue) of the 129Xe nucleus, transferred from the optically polarised Rb electron, are shown. The depicted complex is disturbed by several intruding gas molecules, which change the dynamics of the complex and almost lead (around 3500 x 50 fs) to break-up. [Rantaharju, Hanni and Vaara, Phys. Rev. A 102, 032813 (2020)].

The currently investigated topics include polarization transfer simulations from hyperpolarized 1H adduct in an organic substrate to 13C nuclei in the same molecule within the SABRE-Relay hyperpolarization approach, polarization transfer from optically pumped Rb atoms to 129/131Xe nuclei in spin-exchange optical pumping hyperpolarization, spin-spin coupling interactions in noble-gas comagnetometry, as well as 129Xe exchange and hyperpolarization processes in the Xe biosensor complexes.

Movie 2 (a-c): Exchange processes of types 1, 2, and 3 of a 129Xe atom from a Cryptophane A cage in a metadynamics simulation of a prototypic xenon biosensor in an aqueous solution. In a type-1 process, a single water molecule enters the cage and pushes the xenon atom out, in type 2, two water molecules do the same, and in type 3, the type-2 process is preceded by the in-out exchange of additional two water molecules. [Hilla and Vaara, in preparation (2021)].