Calculating optical absorption spectra for large systems using linear-scaling density functional theory

Abstract

A new method for calculating optical absorption spectra within linear-scaling density-functional theory (LS-DFT) is presented, incorporating a scheme for optimizing a set of localized orbitals to accurately represent unoccupied Kohn-Sham states. Three different schemes are compared and the most promising of these, based on the use of a projection operator, has been implemented in a fullyfunctional LS-DFT code. The method has been applied to the calculation of optical absorption spectra for the metal-free phthalocyanine molecule and the conjugated polymer poly(para-phenylene). Excellent agreement with results from a traditional DFT code is obtained.

Computing the Optical Properties of Large Systems

“…Following the same steps as in the ground-state density matrix optimisation procedure, the conduction optimisation thus yields a density matrix spanning a predefined set of low energy conduction states expressed by a set of NGWFs that is optimised to be an ideal representation for those states. Together, {χ α } and {φ β } then form an ideal representation for both the valence states and a low energy subset of the conduction space and Kohn-Sham energies produced from the two representations are in very good agreement with plane-wave results [109].…”

“…A technique to achieve this within the ONETEP code was recently introduced by Ratcliff and coworkers [109] and will be briefly outlined here.…”

Large-scale density functional theory simulation of inorganic nanotubes: a case study on Imogolite nanotubes

“…As a result, the description of empty KS states in terms of both energy and real-space amplitude, obtained from the standard optimisation of valence NGWFs, can be extremely poor, especially for higher energy empty KS states. 86 To overcome this limitation, which prevents sound calculation of KS DFT band gaps, and extension towards time-dependent (TD-DDFT) or perturbative post-DFT GW approaches, 87 ONETEP implements a procedure for the optimisation of a second set of atom-centred conduction-NGWFs (x a ) to correctly compute empty KS from pre-optimised valence NGWFS (W a ) and ensuing density matrix [r(r, r9)]. The approach, described in Ref.…”

Predicting solvatochromic shifts and colours of a solvated organic dye: The example of nile red

“…This allows for the NGWFs to be optimised in situ during a calculation to ideally represent the ground state density matrix of the system, thus only requiring a minimal number of localised functions to yield a representation of the occupied Kohn-Sham space to the same accuracy as plane-wave codes [38,39]. The ONETEP formalism has been extended to the treatment of unoccupied Kohn-Sham states, where a second set of NGWFs {χ β } is optimised in order to yield an ideal representation of a low energy subset of the conduction space [47]. Together, {χ β } and {φ α } form a very efficient representation for excited states calculations using TDDFT [22,23].…”