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.