The fitted PIM potential is used to investigate the speciation and association constants of La-Cl complexes at 773 K and 5 kbar. The simulation boxes used to study speciation and thermodynamics are compiled in Table (boxes #4 to #7). The edge lengths of the boxes are adjusted to match the densities obtained from EOS in Driesner et al. . To prepare those boxes, first a box containing only water molecules is equilibrated at a density of ∼0.8 g/cm 3 with the SPC/E water model .