NCC: A New, Nonorthogonally Spin-Adapted CCSDTQ Program for Closed Shell Systems

A new CCSDTQ program has been developed as part of the CFOUR program suite, which takes advantage of nonorthogonal spin adaptation using the orbital, or skeleton amplitudes and their spin-summed extensions. An overview of the nonorthogonal spin adaptation theory and spin-summation procedure is given, and the benefits of this method with regards to non-spin-summed (Goldstone), orthogonally spin-adapted and spin-integrated methods are discussed. Timings of the new program are presented and analyzed.