Recently it has become possible to calculate accurate excited state properties of large model systems in complex environments, up to many 1000s of atoms, with linear scaling time-dependent density functional theory (TDDFT). This can be applied to diverse topics such as solvated molecules, nanostructures and biologically inspired molecules, where both the necessary system-size and the number of relevant electronic excitations can be very large. Our approach addresses both problems, and removes the need for explicit representation of Kohn-Sham states, through the use of a minimal, <i>in-situ</i> optimized, local basis representation in terms of which transitions and excitations can be represented as density matrices which can be made sparse. Manipulation of the sparsity pattern of the density matrices also allows us to block unwanted transitions, for example, spurious solvent-solvent excitations in calculations of solvated molecules with many solvent molecules.<br/><br/>We now show how this methodological framework may be extended to calculate excited state properties involving derivatives, including forces, while maintaining linear-scaling with system size. We demonstrate this through large-scale calculations on molecules including photo-active dyes, UV protectant molecules, and pigment-protein complexes. Many exhibit photoisomerism, which can be explored using excited state forces and geometry optimizations. Longer-term, the formalism promises the ability to calculate accurate excited state absorption and emission spectra and excited state dynamics for chromophores in complex environments. For example, one can achieve a high degree of convergence of solvatochromic shifts in absorption and emission spectra with respect to model size and number of snapshots, which would be impossible with the computational scaling of traditional approaches to TDDFT.

Linear scaling excited state forces within the linear-response time dependent DFT formalism: Method and applications

2019 Spring National Meeting

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