FLORENCE MEETS fiRelax – IMPLEMENTATION OF PARAMAGNETIC RELAXATION MODELS INTO A NOVEL WEB-BASED FITTING PLATFORM

Fitting Nuclear Magnetic Relaxation Dispersion (NMRD) profiles reveals structural, dynamic and electronic parameters of a system. In paramagnetic molecules and nanoparticles, water proton relaxation often combines inner-, second- and outer-sphere contributions and fast internal mobility, while systems with static Zero-Field-Splitting (ZFS) are better described by the Florence [1] and modified Florence [2] models. However, no software yet combines a robust fitting algorithm with an accessible library of paramagnetic relaxation models. We present fiRelax (https://firelax.cerm.unifi.it), a web application with a dynamic graphical interface for analyzing experimental profiles and simulating relaxation curves, usable without programming or theoretical expertise. Its Java core ensures stability, while calculations and minimization run on the open-source One-Fit-Engine [3] (OFE), written in Raku and C and used by other fitting programs. Porting the Florence model from Fortran into OFE's C environment has made fitting faster than the original Fortran minimization. The platform has been tested on datasets with multiple relaxation contributions, and supports simultaneous fitting of multiple datasets through shared parameters or Arrhenius-based temperature dependences.