(Heisenberg) exchange and electrostatic interactions between O2 molecules: An a b i n i t i o study

The electrostatic and exchange interactions between two ground state 3Σ−g O2 molecules have been calculated ab initio by means of first order exchange perturbation theory. The nonorthogonality problem has been handled in a second-quantized hole-particle formalism by a generalization of Wick’s theorem. The splitting between the spin states, S=0, 1, and 2, of the O2–O2 dimer is accurately represented by the Heisenberg Hamiltonian. By means of a spherical expansion for the orientational dependence and exponential functions for the distance dependence of the expansion coefficients, complete analytic potential surfaces have been evaluated, both for the spin-independent term in the Heisenberg Hamiltonian ∼(ΔE) and for the exchange coupling parameter J. The strong anisotropy and distance dependence of J indicate that magnon–libron and magnon–phonon coupling in solid O2 are likely to be strong. A simple four-electron model containing the O2 open shells only reproduces the structure dependence of J qualitatively, but not quantitatively.