The charge-transfer-to-solvent (CTTS) absorption band can be used as a sensitive spectroscopic handle for probing short-range solvation structure in liquids, and has recently been isolated for iodide at the air/water interface. This reaction, summarized as I–(aq)+hν→I●(aq)+e–(aq) is the starting point for many downstream chemical processes, with the most straightforward being hydrated electron formation. Time-dependent density functional theory (TDDFT) is a cost-effective method for computing electronic spectra, which is capable of 0.2–0.3 eV accuracy at visible and UV wavelengths. When it comes to solution-phase spectra, however, TDDFT based on semilocal functionals is plagued by spurious low-lying charge-transfer (CT) states that grow in number as the system grows in size, adding significantly to the cost of the calculation and obscuring the physically meaningful excited states. Absent additional modifications, this problem effectively renders TDDFT useless for solution-phase photochemical simulations. Here, we introduce an automated diabatization that allows for fast and effective screening of the CTTS excitation space in solution. With no parameters beyond a specification of which atoms constitute the “chromophore” and which are the “environment”, our procedure defines “natural CT orbitals” and identifies characteristic orbital pairs that are most likely to participate in a CTTS excitation. The CTTS states obtained from this procedure are projected onto the solvent, yielding a fully quantum-mechanical (and automated) criterion for defining the most important solvent molecules in a given excitation. The method is applied to an ab initio molecular dynamics trajectory of I–(aq) in order to define a subspace for projector-based embedding and thereby perform a hybrid-TDDFT-in-semilocal-DFT embedding procedure for solution-phase photochemistry. Our results show quantitative agreement with experiment while only requiring that ~15% of the water molecules be treated at the higher level of theory. This method provides a much-needed theoretical tool to compliment experimental investigations of CTTS spectra of anions at the air/water interface

Finding a needle in a haystack: Isolating the physical charge-transfer states in solution-phase TDDFT

ACS Spring Meeting 2021

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