Reduced density matrix theory

VARIATIONAL TWO-ELECTRON REDUCED-DENSITY-MATRIX THEORY... 

G. Gidofalvi and D. A. Mazziotti, Variational reduced-density-matrix theory strength of Hamiltonian-dependent positivity conditions. Chem. Phys. Lett. 398, 434 (2004). 

J. R. Hammond and D. A. Mazziotti, Variational two-electron reduced-density-matrix theory partial 3-positivity conditions for A-representability. Phys. Rev. A 71, 062503 (2005). 

R. M. Erdahl, Two algorithms for the lower bound method of reduced density matrix theory. Reports Math. Phys. 15, 147-162 (1979). 

M. D. Benayoun and A. Y. Lu, Invariance of the cumulant expansion under l-particle unitary transformations in reduced density matrix theory. Chem. Phys. Lett. 387, 485 (2004). 

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