  We extend a previously proposed rotation and truncation scheme to optimize
quantum Anderson impurity calculations with exact diagonalization [PRB 90,
085102 (2014)] to density-matrix renormalization group (DMRG) calculations. The
method reduces the solution of a full impurity problem with virtually unlimited
bath sites to that of a small subsystem based on a natural impurity orbital
basis set. The later is solved by DMRG in combination with a
restricted-active-space truncation scheme. The method allows one to compute
Green's functions directly on the real frequency or time axis. We critically
test the convergence of the truncation scheme using a one-band Hubbard model
solved in the dynamical mean-field theory. The projection is exact in the limit
of both infinitely large and small Coulomb interactions. For all parameter
ranges the accuracy of the projected solution converges exponentially to the
exact solution with increasing subsystem size.
