Curvy steps for density matrix based energy minimization: tensor formulation and toy applications

A unitary transformation-based framework is explained for varying the one-particle density matrix in a non-orthogonal expansion basis while preserving both the idempotency and electron number constraints. The equations are presented in a compact tensor formulation that permits the use of any representation for the matrices. A connection with the LNV method is established. Curvy steps, which are high order line searches along a chosen descent direction, are defined. Calculations with two toy model systems are presented to illustrate the nature of the displacements in this approach.

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