As mentioned in the last subsection, the convergence threshold of the single-precision sweep is around 10 -4 -10 -5 Hartree, which means that the overall accuracy of the single-precision DMRG method is around 10 -4 -10 -5 Hartree. Therefore, if higher accuracy is required (which is common in DMRG calculations), the following clean-up sweeps, i.e. the double-precision sweep, must be conducted, undermining the overall acceleration.