Well-tempered metadynamics (MTD) simulations are used to sample the free energy landscape of the system for ions pairing; (iii) FPMD simulations refine the structure of the solvation shell and provide molecular configurations to compute UV-visible absorption spectra at finite temperature; (iv) TDDFT is used to compute the absorption spectra of the ions with a hybrid solvation model including their first solvation shell of explicit water molecules and a conductor-like polarizable continuum embedding (CPCM).