16 research outputs found

    Linear-scaling time-dependent density-functional theory beyond the Tamm-Dancoff approximation: Obtaining efficiency and accuracy with in situ optimised local orbitals.

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    We present a solution of the full time-dependent density-functional theory (TDDFT) eigenvalue equation in the linear response formalism exhibiting a linear-scaling computational complexity with system size, without relying on the simplifying Tamm-Dancoff approximation (TDA). The implementation relies on representing the occupied and unoccupied subspaces with two different sets of in situ optimised localised functions, yielding a very compact and efficient representation of the transition density matrix of the excitation with the accuracy associated with a systematic basis set. The TDDFT eigenvalue equation is solved using a preconditioned conjugate gradient algorithm that is very memory-efficient. The algorithm is validated on a small test molecule and a good agreement with results obtained from standard quantum chemistry packages is found, with the preconditioner yielding a significant improvement in convergence rates. The method developed in this work is then used to reproduce experimental results of the absorption spectrum of bacteriochlorophyll in an organic solvent, where it is demonstrated that the TDA fails to reproduce the main features of the low energy spectrum, while the full TDDFT equation yields results in good qualitative agreement with experimental data. Furthermore, the need for explicitly including parts of the solvent into the TDDFT calculations is highlighted, making the treatment of large system sizes necessary that are well within reach of the capabilities of the algorithm introduced here. Finally, the linear-scaling properties of the algorithm are demonstrated by computing the lowest excitation energy of bacteriochlorophyll in solution. The largest systems considered in this work are of the same order of magnitude as a variety of widely studied pigment-protein complexes, opening up the possibility of studying their properties without having to resort to any semiclassical approximations to parts of the protein environment.T.J.Z. acknowledges the support of EPSRC under Grant No. EP/J017639/1 and the ARCHER eCSE programme. M.C.P. and P.D.H. acknowledge the support of EPSRC under Grant No. EP/J015059/1. The underlying data of this publication can be accessed via the following persistent URI: https://www.repository.cam.ac.uk/handle/1810/251293This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493628

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

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    The solvatochromic shift, as well as the change in colour of the simple organic dye nile red, is studied in two polar and two non-polar solvents in the context of large-scale time-dependent density-functional theory (TDDFT) calculations treating large parts of the solvent environment from first principles. We show that an explicit solvent representation is vital to resolve absorption peak shifts between nile red in n-hexane and toluene, as well as acetone and ethanol. The origin of the failure of implicit solvent models for these solvents is identified as being due to the strong solute-solvent interactions in form of π-stacking and hydrogen bonding in the case of toluene and ethanol. We furthermore demonstrate that the failures of the computationally inexpensive Perdew-Burke-Ernzerhof (PBE) functional in describing some features of the excited state potential energy surface of the S1 state of nile red can be corrected for in a straightforward fashion, relying only on a small number of calculations making use of more sophisticated range-separated hybrid functionals. The resulting solvatochromic shifts and predicted colours are in excellent agreement with experiment, showing the computational approach outlined in this work to yield very robust predictions of optical properties of dyes in solution

    Solvent Effects on Electronic Excitations of an Organic Chromophore.

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    In this work we study the solvatochromic shift of a selected low-energy excited state of alizarin in water by using a linear-scaling implementation of large-scale time-dependent density functional theory (TDDFT). While alizarin, a small organic dye, is chosen as a simple example of solute-solvent interactions, the findings presented here have wider ramifications for the realistic modeling of dyes, paints, and pigment-protein complexes. We find that about 380 molecules of explicit water need to be considered in order to yield an accurate representation of the solute-solvent interaction and a reliable solvatochromic shift. By using a novel method of constraining the TDDFT excitation vector, we confirm that the origin of the slow convergence of the solvatochromic shift with system size is due to two different effects. The first factor is a strong redshift of the excitation due to an explicit delocalization of a small fraction of the electron and the hole from the alizarin onto the water, which is mainly confined to within a distance of 7 Å from the alizarin molecule. The second factor can be identified as long-range electrostatic influences of water molecules beyond the 7 Å region on the ground-state properties of alizarin. We also show that these electrostatic influences are not well reproduced by a QM/MM model, suggesting that full QM studies of relatively large systems may be necessary in order to obtain reliable results.TJZ acknowledges the support of EPSRC Grant EP/J017639/1 and funding under the embedded CSE programme of the ARCHER UK National Supercomputing Service. MCP and PDH acknowledge the support of EPSRC grant EP/J015059/1.This is the final version of the article. It first appeared from the American Chemical Society via https://doi.org/10.1021/acs.jctc.5b0101

    Photophysics and Photochemistry of DNA Molecules: Electronic Excited States Leading to Thymine Dimerization

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    We combine quantified natural transition orbital (QNTO) analysis with large-scale linear response time-dependent density functional theory to investigate the concerted [2 + 2] thymine dimerization reaction. This reaction is a main cause of UV-light-induced damage to DNA, but its mechanism has remained poorly understood. QNTO analysis enables the electronic excitations of a molecule to be identified on the basis of their transition origins across a wide range of molecular geometries, allowing the participating excited states to be identified relatively straightforwardly. We identify a barrierless funnel that is responsible for the ultrafast reaction previously indicated in experiments. The reactive state is found to have crossings with several bright excited states, revealing how the initially populated bright states can decay rapidly to the reactive state. We also examine the contribution of environmental factors, such as inclusion of the DNA backbone, which can affect the conformation of the potential energy surfaces of the relevant states
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