, where 𝑟 6 is a distance between i-th Cl* atom and phosphorus atom bonded to the Cl* atom. The 𝑘 4$$5 and 𝑟 7 are the force constant and the equilibrium distance of the harmonic potential, respectively. The 𝑘 4$$5 value was set to 4.48 × 10 3 kcal/ (mol Å 2 ) (2.0 au) as previously reported. On the other hand, the keep angle potential is a harmonic potential on the angle between a P-Cl* bond and a direction of the lone pair orbital, which is formulated as follows: