Original sentence: Although fewer N-electron functions are required if one employs a basis of spin-adapted functions ( eigenfunctions), the most efficient full CI programs employ a Slater determinant basis because this allows for the very rapid evaluation of coupling coefficients using string-based techniques advanced by Nicholas C.In the 1980s and 1990s, full CI programs were adapted to provide arbitrary-order Møller-Plesset perturbation theory wave functions, and in the 2000s they have been adapted to provide coupled cluster wave functions to arbitrary orders, greatly simplifying the task of programming these complex methods.This is particularly important in cases such as bond-breaking reactions, diradicals, and first-row transition metals, where electronic near-degeneracies can invalidate the approximations inherent in many standard methods such as Hartree-Fock theory, multireference configuration interaction, finite-order Møller-Plesset perturbation theory, and coupled cluster theory
Edited sentence: Although fewer N-electron functions are required if one employs a basis of spin-adapted functions (Ŝ2 eigenfunctions), the most efficient full CI programs employ a Slater determinant basis because this allows for the very rapid evaluation of coupling coefficients using string-based techniques advanced by Nicholas C.In the 1980s and 1990s, full CI programs were adapted to provide arbitrary-order Møller–Plesset perturbation theory wave functions, and in the 2000s they have been adapted to provide coupled cluster wave functions to arbitrary orders, greatly simplifying the task of programming these complex methods.This is particularly important in cases such as bond-breaking reactions, diradicals, and first-row transition metals, where electronic near-degeneracies can invalidate the approximations inherent in many standard methods such as Hartree–Fock theory, multireference configuration interaction, finite-order Møller–Plesset perturbation theory, and coupled cluster theory