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

The anisotropic intermolecular potential V(r, theta (1), theta (2), phi )of two non-polar molecules, with its arbitrary coefficients represented by th e functions of the intermolecular distance r, is formulated and analyticall y minimized in the angular variables theta (1), theta (2) and phi = phi (1) - phi (2). In terms of multipole expansion, the explicit form of these coe fficients 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 foun d and equilibrium orientational states can be characterized at each value o f r. An analysis of literature data for the H-2, N-2, and O-2 dimers togeth er with ab initio calculations of V (r, theta (1), theta (2), phi) for the H-2 and N-2 dimers (performed by the MP2 and CCSD(T) methods with the cc-au g-pvqz basis set) demonstrate that the analytical form proposed for the ang ular dependence of V and the result of its minimization are helpful in reve aling intermediate orientational phases in which angular variables change c ontinuously with r, and the function V (r) has regions with a mild slope. O ne such phase with theta (1) = theta (2), phi = 0 has been found to corresp ond to the global minimum of the N-2 dimer.