Explicitly correlated second-order Møller–Plesset perturbation theory for unrestricted Hartree–Fock reference functions with exact satisfaction of cusp conditions

We formulated and implemented explicitly correlated second-order Moller-Plesset perturbation theory for unrestricted Hartree-Fock reference functions with exact satisfaction of the cusp conditions. For this purpose the geminal basis was augmented by spin-flipped functions. Numerical tests for the correlation energies of several open shell systems have shown faster convergence toward the complete basis set limit when the spin-flipped geminals are included. We also performed benchmark calculations of atomization energies for a set of 16 molecules.