Constraint-based, Single-point Approximate Kinetic Energy Functionals

TL;DR

This paper develops an improved orbital-free kinetic energy functional for molecular dynamics, removing unphysical singularities and ensuring accuracy and simplicity based on DFT constraints, suitable for realistic simulations.

Contribution

It introduces a new constraint-based, local kinetic energy functional with higher-order derivatives to eliminate singularities, enhancing accuracy for molecular simulations.

Findings

Functional successfully removes nuclear singularities.

Demonstrated effectiveness on dissociation energy calculations.

Maintains simplicity and accuracy for multi-scale simulations.

Abstract

We present a substantial extension of our constraint-based approach for development of orbital-free (OF) kinetic-energy (KE) density functionals intended for the calculation of quantum-mechanical forces in multi-scale molecular dynamics simulations. Suitability for realistic system simulations requires that the OF-KE functional yield accurate forces on the nuclei yet be relatively simple. We therefore require that the functionals be based on DFT constraints, local, dependent upon a small number of parameters fitted to a training set of limited size, and applicable beyond the scope of the training set. Our previous "modified conjoint" generalized-gradient-type functionals were constrained to producing a positive-definite Pauli potential. Though distinctly better than several published GGA-type functionals in that they gave semi-quantitative agreement with Born-Oppenheimer forces from full Kohn-Sham results, those modified conjoint functionals suffer from unphysical singularities at the nuclei. Here we show how to remove such singularities by introducing higher-order density derivatives. We give a simple illustration of such a functional used for the dissociation energy as a function of bond length for selected molecules.