Quantum-Chemistry-like ¿Nuclear Orbital ¿ approaches as applied to molecular impurities in quantum solvents

Abstract

Helium nanodroplets are applied as an ideal cryogenic matrix for high-resolution spectroscopic studies of trapped molecules, highlighting the key role of Bose-Einstein (or Fermi-Dirac) statistical effects of the quantum solvent. Accurate simulations have been provided by quantum Monte-Carlo methods. In contrast to the ground-state, excited states in general (and fermionic solvent states in particular) are difficult to address with these methods because one has to deal with the ¿sign problem¿. Alternative Quantum-Chemistry-like ¿Nuclear Orbital¿ approaches (i.e., DFT-based, multi-orbital Hartree, Hartree-Fock, and Full-Configuration-Interaction methods), have being specifically developed and/or implemented to the microscopic description of these systems [3-6]. These methods consider the solvent species as ¿pseudo-electrons¿ and the atoms composing the host molecule as pseudo-nuclei (i.e., replacing Coulomb interactions by solvent-solvent and solvent-dopant pair potentials) so that all of the symmetry (bosonic or fermionic) effects are automatically included and the know-how developed in electronic structure theory can be applied. Illustrative applications will be presented and attention will be paid to the excited states of the quantum solvent species and recent methodological advances. Although our work has been focussed on clusters composed by a molecule and a few quantum solvent species, I will also touch up on the extensions of these methods to clusters in helium nanodrops by connecting with recent research on extended systems with finite cluster approaches.