Quantum many-body dynamics in a Lagrangian frame: II. Geometric formulation of time-dependent density functional theory

TL;DR

This paper reformulates time-dependent density functional theory (TDDFT) in a co-moving Lagrangian frame, simplifying the description of many-body dynamics and enabling local nonadiabatic exchange-correlation functionals.

Contribution

It introduces a Lagrangian-based formulation of TDDFT using deformation and vorticity tensors, resolving nonlocality issues and facilitating practical local approximations.

Findings

Lagrangian formulation simplifies TDDFT equations.

Derived local nonadiabatic exchange-correlation functionals.

Examples demonstrate practical applicability of new functionals.

Abstract

We formulate equations of time-dependent density functional theory (TDDFT) in the co-moving Lagrangian reference frame. The main advantage of the Lagrangian description of many-body dynamics is that in the co-moving frame the current density vanishes, while the density of particles becomes independent of time. Therefore a co-moving observer will see the picture which is very similar to that seen in the equilibrium system from the laboratory frame. It is shown that the most natural set of basic variables in TDDFT includes the Lagrangian coordinate, $\bm\xi$, a symmetric deformation tensor $g_{\mu\nu}$, and a skew-symmetric vorticity tensor, $F_{\mu\nu}$. These three quantities, respectively, describe the translation, deformation, and the rotation of an infinitesimal fluid element. Reformulation of TDDFT in terms of new basic variables resolves the problem of nonlocality and thus allows to regularly derive a local nonadiabatic approximation for exchange correlation (xc) potential. Stationarity of the density in the co-moving frame makes the derivation to a large extent similar to the derivation of the standard static local density approximation. We present a few explicit examples of nonlinear nonadiabatic xc functionals in a form convenient for practical applications.