Intermolecular potential and equilibrium orientational states for dimers of non-polar molecules

Abstract

The anisotropic intermolecular potential V(r ⊘₁ ⊘₂ ϕ) of two non-polar molecules, with its arbitrary coefficients represented by the functions of the intermolecular distance r, is formulated and analytically minimized in the angular variables ⊘₁, ⊘₂ and ϕ = ϕ₁ −ϕ₂. In terms of multipole expansion, the explicit form of these coefficients is strictly specified by the constants of quadrupole, dispersion, and repulsive interactions of two non-polar molecules. These coefficients are also obtainable from basic orientational configurations computed by ab initio methods. As a result, the intermolecular potential V(r) can be found and equilibrium orientational states can be characterized at each value of r. An analysis of literature data for the H₂, N₂, and O₂ dimers together with ab initio calculations of V(r, ⊘₁, ⊘₂ ϕ) for the H₂ and N₂ dimers (performed by the MP2 and CCSD(T) methods with the cc-aug-pvqz basis set) demonstrate that the analytical form proposed for the angular dependence of V and the result of its minimization are helpful in revealing intermediate orientational phases in which angular variables change continuously with r, and the function V(r) has regions with a mild slope. One such phase with ⊘₁ = ⊘₂, ϕ = 0 has been found to correspond to the global minimum of the N₂ dimer.