Open-shell explicitly correlated F12 methods

In calculations on open-shell systems, spin-flipped geminals must be used in explicitly correlated F12 methods with spin-unrestricted or restricted open-shell Hartree-Fock reference wave functions. We suggest the use of the sp ansatz of Bokhan et al. [J. Chem. Phys. 131, 084105 (2009)] for calculations with fixed F12 amplitudes, but to use a novel contracted geminal approach if the F12 amplitudes are to be optimised. This new approach is denoted MP2-F12-o (CCSD(F12)-o). The performance of the new approach is assessed by calculating the atomisation energies of a test set comprising 106 molecules containing the atoms H, C, N, O and F, and by calculating the ionisation energies and electron affinities of the atoms C, N, O and F.