Calculation of zero field splitting parameters for trimethylenemethane

The spin dipole–dipole contribution to the zero field splitting of the 3A′2 ground state of trimethylenemethane has been computed with ab initio CI wave functions which included selected single and double excitations from the Hartree–Fock configuration. The resulting value of D is 0.020 cm−1. Isotropic and anisotropic proton hyperfine coupling constants were also computed. The difficulty of quantitatively reproducing experimental values for these properties is discussed.