Molecular dynamics simulation of the order-disorder phase transition in solid NaNO₂

We present molecular dynamics simulations of solid NaNO₂ using pair potentials with the rigid-ion model. The crystal potential surface is calculated by using an a priori method which integrates the ab initio calculations with the Gordon-Kim electron gas theory. This approach is carefully examined by using different population analysis methods and comparing the intermolecular interactions resulting from this approach with those from the ab initio Hartree-Fock calculations. Our numerics show that the ferroelectric-paraelectric phase transition in solid NaNO₂ is triggered by rotation of the nitrite ions around the crystallographical c axis, in agreement with recent x-ray experiments [Gohda et al., Phys. Rev. B 63, 14101 (2000)]. The crystal-field effects on the nitrite ion are also addressed. An internal charge-transfer effect is found.