Ab initio study of the magnetic interactions in the spin-ladder compound SrCu2O3

A wide range of experimental, semiempirical, and theoretical values have been reported in the literature for the magnetic coupling parameters of the mio-leg ladder compound SrCu2O3. We apply quantum chemical and density functional techniques to calculate accurate N-electron wave functions or densities for two different Cu2O7 clusters that represent the leg (J(\\)) and rung (J(perpendicular to)) Of the ladder. Our data indicate that J(\\) is slightly larger than J(perpendicular to) (J(perpendicular to)/J(\) approximate to 0.9) with J(perpendicular to) = -139 meV (-1670 K) and J(\\) = -156 meV (-1870 K). Recent experimental data indicate a more strongly anisotropic ratio, J(perpendicular to)/J\\approximate to 0.5. The origin of the difference is unclear, as our ab initio estimates of J(perpendicular to) and J(\\) seem to be converged with respect to the size of the basis set, the level of electron correlation, and the size of the cluster. However, we also find a surprisingly strong ferromagnetic interladder interaction which may play a role in resolving this discrepancy. [S0163-1829(99)01126-1].