Semiempirical LCAO calculations of electronic and dynamical properties of α and γ nitrogen crystals and nitrogen aggregates

Abstract

Semi-empirical LCAO methods (extended Huckel, iterative extended Huckel and INDO) are examined as a possible simple starting point for constructing solid state interaction potentials for non-bonded interactions from molecular wave functions. These methods are applied to calculate the cohesion energy and lattice equilibrium unit cell dimensions for α and γ nitrogen crystals, the lattice optical mode frequencies at = 0 of α - N₂, the electronic Davydov splitting of the molecular ¹Π_g state in α - N₂ and the stability of molecular-ion aggregates. Reasonable agreement is obtained with extended Huckel and iterative extended Huckel methods, while INDO usually yields poor results. The possibility of excimer formation in nitrogen is discussed in light of these calculations. It is concluded that semiquantitative results can be obtained with these methods when they include the gross features essential for description of crystal non-bonded interactions, such as full overlap interactions, approximate magnitude of molecular permanent quadrupole moment, orthogonality corrections and self-consistent charge transfer. However, when more accurate results are required, ab initio methods cannot be avoided.