Table shows the results for water clusters. Pure single precision calculations (without the clean-up step) do not introduce significant numerical errors in total energies, i.e. the typical difference between single and double precision energies is only several J/mol, which is three orders of magnitude below chemical accuracy. Moreover, the double-precision numerical accuracy is fully recovered when the cleanup iteration is performed. The single-precision calculation is twice as fast than the double-precision one. For the calculation with the clean-up step, the speedup quickly approaches the theoretical maximum (about 1.5, for these parameters) with the increasing number of water molecules (see Fig. ). We observed similar performance and accuracy for RI/CD-CCSD calculations of these clusters. To test whether numerical errors increase with system size, we compared single and double precision CCSD for an adenine-thymine-thymine system (ATT) and found that the difference between single and double precision total energies is 15 J/mol (again, full double precision accuracy can be recovered with a single clean-up step).