An effective solution to convex $1$-body $N$-representability

• URL: http://arxiv.org/abs/2105.06459v2
• Date: Wed, 9 Jun 2021 10:54:38 GMT
• Title: An effective solution to convex $1$-body $N$-representability
• Authors: Federico Castillo and Jean-Philippe Labb\'e and Julia Liebert and Arnau Padrol and Eva Philippe and Christian Schilling
• Abstract summary: We study the generalization of the $1$-body $N$-representability problem to ensemble states with fixed spectrum $mathbfw$. We adapt and further develop tools such as symmetric polytopes, sweep polytopes, and Gale order.
• Score: 0.0
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: From a geometric point of view, Pauli's exclusion principle defines a hypersimplex. This convex polytope describes the compatibility of $1$-fermion and $N$-fermion density matrices, therefore it coincides with the convex hull of the pure $N$-representable $1$-fermion density matrices. Consequently, the description of ground state physics through $1$-fermion density matrices may not necessitate the intricate pure state generalized Pauli constraints. In this article, we study the generalization of the $1$-body $N$-representability problem to ensemble states with fixed spectrum $\mathbf{w}$, in order to describe finite-temperature states and distinctive mixtures of excited states. By employing ideas from convex analysis and combinatorics, we present a comprehensive solution to the corresponding convex relaxation, thus circumventing the complexity of generalized Pauli constraints. In particular, we adapt and further develop tools such as symmetric polytopes, sweep polytopes, and Gale order. For both fermions and bosons, generalized exclusion principles are discovered, which we determine for any number of particles and dimension of the $1$-particle Hilbert space. These exclusion principles are expressed as linear inequalities satisfying hierarchies determined by the non-zero entries of $\mathbf{w}$. The two families of polytopes resulting from these inequalities are part of the new class of so-called lineup polytopes.

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