@misc{indiciaeebe72388b2a3,
  title = {Density matrix perturbation theory.},
  author = {Anders M N Niklasson and Matt Challacombe},
  year = {2004},
  doi = {10.1103/physrevlett.92.193001},
  url = {https://pubmed.ncbi.nlm.nih.gov/15169398/},
  note = {Source identifier: 15169398}
}