For bound states of atoms and molecules we obtain decay estimates for the eigenvalues of the K-particle reduced density matrix. Previously, eigenvalue bounds were obtained by A.V. Sobolev in the K = 1 case and, by extension, the K = N − 1 case. The purpose of the current work is to obtain bounds for the remaining cases.
The expectation value of the Hamiltonian H for a many-electron system may be expressed in terms of the reduced second-order density matrix and the “reduced” Hamiltonian K for a two-electron system. It may be shown that the antisymmetric component HAS of the original Hamiltonian is an “outer projection” of K with respect to the antisymmetrizer O, so that HAS = OKO. This result implies that the eigenvalues of K are, in order, lower bounds to the eigenvalues of HAS, i.e. to the eigenvalues of H associated with the antisymmetry requirement. The expectation values of K are hence often below the ground state energy of H, and the eigenvalues may be used for the calculation of lower bounds. Some numerical applications to atoms serve as an illustration, and it is shown that the lower bounds become worse as the number of electrons increases. The implications for the “representability problem” are discussed.
Abstract We prove that the Hilbert–Schmidt norm of k -particle reduced density matrices of N -body fermionic states is bounded from above by CkNk/2 C k N k / 2 - matching the scaling behaviour of Slater determinant states. This generalises a recent result of Christiansen [3] on 2-particle reduced density matrices to higher order density matrices. Moreover, our estimate directly yields a lower bound on the von Neumann entropy and the 2-Rényi entropy of reduced density matrices, thereby providing further insight into conjectures of Carlen–Lieb–Reuvers [2, 8].
It is shown that the eigenvalues λk,k=1,2,…, of the one-particle density matrix satisfy the bound λk≤Ck−8/3 with a positive constant C.
The least upper bound of the eigenvalues of second-order reduced density matrices for a system of fermions is proved to be n for a system of 2n or 2n+1 identical fermions. It is also shown that this limiting state may be interpreted as a system of identical pairs behaving as quasibosons.