Colloidal quantum dots (QDs) are near-ideal nanomaterials for energy conversion and lighting technologies. However, their photophysics exhibits supreme sensitivity to surface passivation and defects, of which control is problematic. The role of passivating ligands in photodynamics remains questionable and is a focus of ongoing research. The optically forbidden nature of surface-associated states makes direct measurements on them challenging. Therefore, computational modeling is imperative for insights into surface passivation and its impact on light-driven processes in QDs. This Account discusses challenges and recent progress in understanding surface effects on the photophysics of QDs addressed via quantum-chemical calculations. We overview different methods, including the effective mass approximation (EMA), time-dependent density functional theory (TDDFT), and multiconfiguration approaches, considering their strengths and weaknesses relevant to modeling of QDs with a complicated surface. We focus on CdSe, PbSe, and Si QDs, where calculations successfully explain experimental trends sensitive to surface defects, doping, and ligands. We show that the EMA accurately describes both linear and nonlinear optical properties of large-sized CdSe QDs (>2.5 nm), while TDDFT is required for smaller QDs where surface effects dominate. Both approaches confirm efficient two-photon absorption enabling applications of QDs as nonlinear optical materials. TDDFT also describes the effects of morphology on the optical response of QDs: the photophysics of stoichiometric, magic-sized XY (X = Cd, Pb; Y = S, Se) QDs is less sensitive to their passivation compared with nonstoichiometric XY QDs. In the latter, surface-driven optically inactive midgap states can be eliminated by anionic ligands, explaining the better emission of metal-enriched QDs compared with nonmetal-enriched QDs. Ideal passivation of magic-sized QDs by amines and phosphine oxides leaves lower-energy transitions intact, while thiol derivatives add ligand-localized trap states to the band gap. Depending on its position, any loss of ligand from the QD's surface also introduces electron or hole traps, decreasing the QD's luminescence. TDDFT investigations of QD-ligand and QD-QD interactions provide an explanation of experimentally detected enhancement of blinking on-times in closely packed Si QDs and establish favorable conditions for hole transfer from the photoexcited CdSe QD to metal-organic dyes. While TDDFT well describes qualitative trends in optical response to stoichiometry and ligand modifications of QDs, it is unable to calculate highly correlated electronic states like biexcitons and magnetic-dopant-derived states. In these cases, multiconfiguration methods are applied to small nanoclusters and the results are extrapolated to larger-sized QDs, providing reasonable explanations of experimental observables. For light-driven dynamics, the electron-phonon couplings are important, and nonadiabatic dynamics (NAD) is applied. NAD based on first-principles calculations...

First results on electron densities and energies for a number of molecular complexes with different interaction strengths (ranging from ca. 0.3 to 40 kcal/mol), obtained using our recently introduced DFT-in-DFT embedding equations (i.e., Kohn-Sham equations with constrained electron density (KSCED) and external orbital orthogonality (ext orth), KSCED(x, ext orth), where x denotes the single particle support: monomer (m); supermolecular (s); or extended monomer (e)) are compared with densities from supermolecular Kohn-Sham (KS)-DFT calculations and traditional DFT-in-DFT results. Because our methodology does not rely on error-prone potentials that are not present in supermolecular KS-DFT calculations, it allows DFT-in-DFT calculations to achieve much higher accuracy than previous protocols of DFT-in-DFT that employed such potentials. It is shown that whereas conventional DFT-in-DFT embedding theory leads to errors in the electron density at the boundary between subsystems, the situation is remedied when orbital orthogonality between subsystems (i.e., external orthogonality) is enforced. Our approach reproduces KS-DFT total energies at least to the seventh decimal place (and exactly at most geometries) for the tested systems. Potential energy curves (PECs) of the separation of some of the tested systems into fragments are calculated. PECs, obtained with the new equations, using the usual Kohn-Sham equations with constrained electron density and supermolecular basis expansion [KSCED(s, ext orth, v(T) = 0), where v(T) is the nonadditive kinetic potential] were found to be virtually identical to those from conventional KS-DFT; equilibrium distances and interaction energies were reproduced to all reported digits for both local density approximation (LDA) and generalized gradient approximation (GGA) functionals. As an additional approximation, an alternative one-particle space (to the common monomer or supermolecular spaces) in which KS orbitals of a subsystem are expanded is introduced. This expansion, which we refer to as the extended monomer expansion [e.g., KSCED(e)], includes basis functions centered on atom(s) of the complementary subsystem in the interfacial region. Density differences and PECs obtained with the new equations and new one-particle space [i.e., KSCED(e, ext orth, v(T) = 0)] were closely related to those obtained from KSCED(s, ext orth, v(T) = 0). The new approach does not require any supermolecular calculations.

Using density functional theory (DFT) and time-dependent DFT (TDDFT), we investigate the effects of carboxylate groups on the electronic and optical properties of CdSe quantum dots (QDs). We specifically focus on the mechanisms of the binding of the acetate anion to the QD surface with and without excess of Cd(2+) cations. Our calculations show that the most stable ligated conformations are those where an acetate is attached to extra Cd(2+) ion forming a [Cd(2+)(CH3COO(-))] at the QD's surface, while also accompanied by an acetate attached nearby at the surface balancing the overall neutral charge of the system. In contrast, formation of a neutral metal-acetate complex [Cd(2+)(CH3COO(-))2] at the QD surface is found to be the least energetically preferable. A strength of the QD-ligand interaction depends on the solvent, the facet of the QD to which the ligands are attached, and the binding mode - with the bridging mode found to be the most stable conformation for both acetate and cadmium acetate ligands. The cadmium acetate ligands introduce electron trap states at the edge of the conduction band - unoccupied orbitals predominately localized on Cd(2+) ion - that are extremely sensitive to the ligand position and the solvent polarity. Polar solvents like acetonitrile delocalize the electronic density over the entire system and, thus, eliminate trap states. As a result, mixed passivation of the CdSe QDs by pairs of cadmium acetate and acetate ligands provides optimal optical properties with minimal contributions of the ligand-related trap states and optically bright lowest energy transitions.