Per-Olov Löwdin. Quantum Theory of Many-Particle Systems. III. Extension of the Hartree–Fock Scheme to Include Degenerate Systems and Correlation Effects. „Physical Review”. 97 (6), s. 1509–1520, March 1955. American Physical Society. DOI: 10.1103/PhysRev.97.1509. Bibcode: 1955PhRv...97.1509L.
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Per-Olov Löwdin. Quantum Theory of Many-Particle Systems. III. Extension of the Hartree–Fock Scheme to Include Degenerate Systems and Correlation Effects. „Physical Review”. 97 (6), s. 1509–1520, March 1955. American Physical Society. DOI: 10.1103/PhysRev.97.1509. Bibcode: 1955PhRv...97.1509L.