Extensions of the siesta dft code for simulation of molecules

TL;DR

This paper presents new extensions to the siesta DFT code enabling simulation of isolated molecules, their absorption spectra, and improved electrostatic and boundary condition handling, enhancing its capabilities for molecular quantum chemistry.

Contribution

The paper introduces novel features to the siesta DFT code, including multi-grid Poisson solver, pseudo-potential truncation, electrostatic fitting, and integration of time-dependent DFT for absorption spectra.

Findings

Enhanced simulation accuracy for isolated molecules.

Successful implementation of absorption spectra calculations.

Improved boundary condition handling with multipole expansion.

Abstract

We describe extensions to the siesta density functional theory (dft) code [30], for the simulation of isolated molecules and their absorption spectra. The extensions allow for: - Use of a multi-grid solver for the Poisson equation on a finite dft mesh. Non-periodic, Dirichlet boundary conditions are computed by expansion of the electric multipoles over spherical harmonics. - Truncation of a molecular system by the method of design atom pseudo- potentials of Xiao and Zhang[32]. - Electrostatic potential fitting to determine effective atomic charges. - Derivation of electronic absorption transition energies and oscillator stren- gths from the raw spectra produced by a recently described, order O(N3), time-dependent dft code[21]. The code is furthermore integrated within siesta as a post-processing option.