In order to meaningfully compare the differences between single and double precision, one should keep in mind that in these calculations the energy threshold for CCSD convergence was 10 -6 hartree ≈2.6 J/mol. Within this threshold, the error of the calculations with the cleanup step is the same as in the single-precision calculation. Interestingly, calculation of doubleprecision energies from single-precision amplitudes gives larger errors, which can be explained by the convergence criteria used: the amplitudes are underoptimized in comparison with the amplitudes, used for other calculations (shown in Table ).