Fragment Approach to Constrained Density Functional Theory Calculations using Daubechies Wavelets

TL;DR

This paper extends a wavelet-based linear scaling DFT method to include a fragment approach for charge-constrained calculations, enabling efficient and precise modeling of complex molecular systems without reoptimizing basis sets.

Contribution

It introduces a fragment-based formalism within a wavelet DFT framework that reuses support functions for charge constraints, enhancing efficiency and flexibility.

Findings

Supports highly precise charge-constrained DFT calculations

Enables reuse of basis sets across different configurations

Facilitates modeling of systems in complex environments

Abstract

In a recent paper we presented a linear scaling Kohn-Sham density functional theory (DFT) code based on Daubechies wavelets, where a minimal set of localized support functions is optimized in situ and therefore adapted to the chemical properties of the molecular system. Thanks to the systematically controllable accuracy of the underlying basis set, this approach is able to provide an optimal contracted basis for a given system: accuracies for ground state energies and atomic forces are of the same quality as an uncontracted, cubic scaling approach. This basis set offers, by construction, a natural subset where the density matrix of the system can be projected. In this paper we demonstrate the flexibility of this minimal basis formalism in providing a basis set that can be reused as-is, i.e. without reoptimization, for charge-constrained DFT calculations within a fragment approach. Support functions, represented in the underlying wavelet grid, of the template fragments are roto-translated with high numerical precision to the required positions and used as projectors for the charge weight function. We demonstrate the interest of this approach to express highly precise and efficient calculations for preparing diabatic states and for the computational setup of systems in complex environments.