λ‑xTB reorganization energy calculator

1. SMILES → 3D geometry via RDKit ETKDG + MMFF
2. GFN-FF loose pre-optimisation (fast force field)
3. CREST --squick --gfnff conformational search (3 MTD runs) for flexible molecules — returns the lowest-energy conformer
4. Tight GFN2-xTB geometry optimisation of neutral, cation, and anion
5. Four cross single-point energies at GFN2-xTB level
6. Nelsen four-point formula → λ⁺ (hole) and λ⁻ (electron)

Warning: The conformational search is a crude approximation of the global minimum. Results may differ between runs for flexible molecules. For publication-quality values, use proper CREST sampling and DFT (B3LYP/def2-SVP).