Density-potential mappings in quantum dynamics

TL;DR

This paper extends the mathematical proof that the density of a quantum system determines its potential, introduces a numerical fixed-point method, and analyzes convergence conditions in one-dimensional cases.

Contribution

It generalizes the existence and uniqueness theorems in time-dependent density functional theory and provides a numerical implementation with convergence analysis.

Findings

Fixed point iteration converges under certain spectral conditions.

Explicit relation between boundary conditions and convergence.

Numerical example demonstrating the method's effectiveness.

Abstract

In a recent letter [Europhys. Lett. 95, 13001 (2011)] the question of whether the density of a time-dependent quantum system determines its external potential was reformulated as a fixed point problem. This idea was used to generalize the existence and uniqueness theorems underlying time-dependent density functional theory. In this work we extend this proof to allow for more general norms and provide a numerical implementation of the fixed-point iteration scheme. We focus on the one-dimensional case as it allows for a more in-depth analysis using singular Sturm-Liouville theory and at the same time provides an easy visualization of the numerical applications in space and time. We give an explicit relation between the boundary conditions on the density and the convergence properties of the fixed-point procedure via the spectral properties of the associated Sturm-Liouville operator. We show precisely under which conditions discrete and continuous spectra arise and give explicit examples. These conditions are then used to show that in the most physically relevant cases the fixed point procedure converges. This is further demonstrated with an example.