Feature: modified density matrix to overcome local minimum in DMRG

[tanlin2013]

Local minimums are often encountered during the DMRG sweeping, especially for the single-site DMRG. To overcome this issue, it's suggested to bring in the modified density matrix, which considers the small perturbation from the rest of the environment.

References

1. S. R. White, Density matrix renormalization group algorithms with a single center site, Phys. Rev. B 72, 180403 (2005)
2. U. Schollwoeck, The density-matrix renormalization group in the age of matrix product states, arXiv:1008.3477 (2011). (p. 71, eq. 217)