Mechanism for the Enhanced Excited-State Lewis Acidity of Methyl Viologen

Aqueous solutions of methyl viologen (MV²⁺) exhibit anomalous fluorescence behavior. Although it has long fluorescence lifetimes in polar solvents such as acetonitrile, MV²⁺ has a short fluorescence lifetime in water. Recent experiments by Kohler and co-workers (Henrich et al. <i>J. Phys. Chem. B</i> <b>2015</b>, <i>119</i>, 2737–2748) have implicated an excited-state acid/base reaction as the source of the nonradiative decay pathway. While many chemical species exhibit enhanced Brønsted acidity in their excited state, MV²⁺ is the first example of a species with enhanced Lewis acidity. Using a complete active space configuration interaction (CASCI) approach, excited-state molecular dynamics simulations of aqueous MV²⁺ are performed in order to test the hypothesis that MV²⁺ acts as a Lewis photoacid and to elucidate a mechanism for this behavior. These simulations show that the Lewis acidity of MV²⁺ is indeed enhanced by photoexcitation. On its S₁ excited state, MV²⁺ reacts with water to generate a hydronium ion approximately 1.5 ps after excitation. After the hydronium ion is produced, the corresponding hydroxide ion adds to MV²⁺ to form a covalently bound photoproduct and, subsequently, evolves toward a conical intersection

Improved Complete Active Space Configuration Interaction Energies with a Simple Correction from Density Functional Theory

Recent algorithmic advances have extended the applicability of complete active space configuration interaction (CASCI) methods to molecular systems with hundreds of atoms. While this enables simulation of photochemical dynamics in the condensed phase, the underlying CASCI method has some well-known problems resulting from a severe neglect of dynamic electron correlation. Vertical excitation energies, vibrational frequencies, and reaction barriers are systematically overestimated; these errors limit the applicability of CASCI. We develop a correction for the CASCI energy using density functional theory (DFT). The DFT correction incorporates the effect of dynamic electron correlation among the core electrons into the CASCI Hamiltonian. We show that the resulting DFT-corrected CASCI approach is applicable in situations where the usual single-reference DFT methods fail, such as the description of systems with biradicaloid electronic structure and conical intersections between ground and excited electronic states. Finally, we apply this DFT-corrected CASCI approach to ultrafast excited-state proton transfer dynamics. Without the DFT correction, CASCI predicts spurious reaction barriers to these processes, and, as a result, a qualitatively correct description of the dynamics is not possible. With the DFT-corrected CASCI method, we demonstrate qualitative and quantitative agreement with both theory and experiment for two model systems for excited-state intramolecular proton transfer. Finally, we apply the DFT-corrected CASCI method to excited-state proton transfer dynamics in a system with more than 150 atoms

Effect of Nonplanarity on Excited-State Proton Transfer and Internal Conversion in Salicylideneaniline

Salicylideneaniline (SA) is a prototype for excited-state intramolecular proton transfer (ESIPT) reactions in nonplanar molecules. It is generally understood that the dominant photochemical pathway in this molecule is ESIPT followed by nonradiative decay due to twisting about its phenolic bond. However, the presence of a secondary internal conversion pathway resulting from frustrated proton transfer remains a matter of contention. We perform a detailed nonadiabatic dynamics simulation of SA and definitively identify the existence of both reaction pathways, thereby showing the presence of a secondary photochemical pathway and providing insight into the nature of ESIPT dynamics in molecules with nonplanar ground-state geometries

Excited-State Dynamics of a Benzotriazole Photostabilizer: 2‑(2′-Hydroxy-5′-methylphenyl)­benzotriazole

A large number of common photostabilizers are based on the 2-(2′-hydroxyphenyl)­benzotriazole structure. One common example is 2-(2′-hydroxy-5′-methylphenyl)­benzotriazole, or TINUVIN-P. The excited-state dynamics of this molecule have been extensively characterized by ultrafast spectroscopies. These experiments have established that upon photoexcitation TINUVIN-P exhibits excited-state proton transfer followed by a remarkably fast internal conversion. We simulate the excited-state dynamics using <i>ab initio</i> multiple spawning (AIMS) and a complete active space configuration interaction (CASCI) wave function with a correction from density functional theory (DFT) to generate the potential energy surfaces. We predict ultrafast proton transfer on the order of 20 fs followed by simultaneous twisting and pyramidalization until a seam of conical intersection is reached. Near the intersection seam population transfer to the ground state is highly efficient. The process is best described as ballistic wavepacket motion from the Franck–Condon point along a barrierless coordinate leading to the seam of intersection. Internal conversion is primarily mediated by a minimum-energy conical intersection (MECI) with a high degree of pyramidalization. We posit that the presence of a nitrogen atom in the bond linking the phenyl to the benzotriazole allows for the rapid pyramidalization and the short excited-state lifetime

Robust and Efficient Spin Purification for Determinantal Configuration Interaction

The limited precision of floating point arithmetic can lead to the qualitative and even catastrophic failure of quantum chemical algorithms, especially when high accuracy solutions are sought. For example, numerical errors accumulated while solving for determinantal configuration interaction wave functions via Davidson diagonalization may lead to spin contamination in the trial subspace. This spin contamination may cause the procedure to converge to roots with undesired ⟨<i>Ŝ</i>²⟩, wasting computer time in the best case and leading to incorrect conclusions in the worst. In hopes of finding a suitable remedy, we investigate five purification schemes for ensuring that the eigenvectors have the desired ⟨<i>Ŝ</i>²⟩. These schemes are based on projection, penalty, and iterative approaches. All of these schemes rely on a direct, graphics processing unit-accelerated algorithm for calculating the <b>S</b>^<b>2</b><b>c</b> matrix-vector product. We assess the computational cost and convergence behavior of these methods by application to several benchmark systems and find that the first-order spin penalty method is the optimal choice, though first-order and Löwdin projection approaches also provide fast convergence to the desired spin state. Finally, to demonstrate the utility of these approaches, we computed the lowest several excited states of an open-shell silver cluster (Ag₁₉) using the state-averaged complete active space self-consistent field method, where spin purification was required to ensure spin stability of the CI vector coefficients. Several low-lying states with significant multiply excited character are predicted, suggesting the value of a multireference approach for modeling plasmonic nanomaterials

“Balancing” the Block Davidson–Liu Algorithm

We describe a simple modification (“balancing”) of the block Davidson–Liu eigenvalue algorithm which allows the norms of the Krylov search directions to decrease naturally as convergence is approached. In the context of integral-direct configuration interaction singles and time-dependent density functional theory, this provides for efficient utilization of density-based screening. Tests within the TeraChem GPGPU code exhibit speedups of ∼2× on systems with up to 1500 atoms, with negligible loss in accuracy

Nonadiabatic Ab Initio Molecular Dynamics with the Floating Occupation Molecular Orbital-Complete Active Space Configuration Interaction Method

We show that the floating occupation molecular orbital complete active space configuration interaction (FOMO-CASCI) method is a promising alternative to the widely used complete active space self-consistent field (CASSCF) method in direct nonadiabatic dynamics simulations. We have simulated photodynamics of three archetypal molecules in photodynamics: ethylene, methaniminium cation, and malonaldehyde. We compared the time evolution of electronic populations and reaction mechanisms as revealed by the FOMO-CASCI and CASSCF approaches. Generally, the two approaches provide similar results. Some dynamical differences are observed, but these can be traced back to energetically minor differences in the potential energy surfaces. We suggest that the FOMO-CASCI method represents, due to its efficiency and stability, a promising approach for direct ab initio dynamics in the excited state

Accurate Prediction of Noncovalent Interaction Energies with the Effective Fragment Potential Method: Comparison of Energy Components to Symmetry-Adapted Perturbation Theory for the S22 Test Set

Noncovalent interactions play an important role in the stabilization of biological molecules. The effective fragment potential (EFP) is a computationally inexpensive ab initio-based method for modeling intermolecular interactions in noncovalently bound systems. The accuracy of EFP is benchmarked against the S22 and S66 data sets for noncovalent interactions [Jurečka, P.; Šponer, J.; Černý, J.; Hobza, P. <i>Phys. Chem. Chem. Phys.</i> <b>2006</b>, <i>8</i>, 1985; Řezáč, J.; Riley, K. E.; Hobza, P. <i>J. Chem. Theory Comput.</i> <b>2011</b>, <i>7</i>, 2427]. The mean unsigned error (MUE) of EFP interaction energies with respect to coupled-cluster singles, doubles, and perturbative triples in the complete basis set limit [CCSD­(T)/CBS] is 0.9 and 0.6 kcal/mol for S22 and S66, respectively, which is similar to the MUE of MP2 and SCS-MP2 for the same data sets, but with a greatly reduced computational expense. Moreover, EFP outperforms classical force fields and popular DFT functionals such as B3LYP and PBE, while newer dispersion-corrected functionals provide a more accurate description of noncovalent interactions. Comparison of EFP energy components with the symmetry-adapted perturbation theory (SAPT) energies for the S22 data set shows that the main source of errors in EFP comes from Coulomb and polarization terms and provides a valuable benchmark for further improvements in the accuracy of EFP and force fields in general

Quantum-Mechanical Analysis of the Energetic Contributions to π Stacking in Nucleic Acids versus Rise, Twist, and Slide

Symmetry-adapted perturbation theory (SAPT) is applied to pairs of hydrogen-bonded nucleobases to obtain the energetic components of base stacking (electrostatic, exchange-repulsion, induction/polarization, and London dispersion interactions) and how they vary as a function of the helical parameters Rise, Twist, and Slide. Computed average values of Rise and Twist agree well with experimental data for B-form DNA from the Nucleic Acids Database, even though the model computations omitted the backbone atoms (suggesting that the backbone in B-form DNA is compatible with having the bases adopt their ideal stacking geometries). London dispersion forces are the most important attractive component in base stacking, followed by electrostatic interactions. At values of Rise typical of those in DNA (3.36 Å), the electrostatic contribution is nearly always attractive, providing further evidence for the importance of charge-penetration effects in π–π interactions (a term neglected in classical force fields). Comparison of the computed stacking energies with those from model complexes made of the “parent” nucleobases purine and 2-pyrimidone indicates that chemical substituents in DNA and RNA account for 20–40% of the base-stacking energy. A lack of correspondence between the SAPT results and experiment for Slide in RNA base-pair steps suggests that the backbone plays a larger role in determining stacking geometries in RNA than in B-form DNA. In comparisons of base-pair steps with thymine versus uracil, the thymine methyl group tends to enhance the strength of the stacking interaction through a combination of dispersion and electrosatic interactions

Tensor Hypercontraction Second-Order Møller–Plesset Perturbation Theory: Grid Optimization and Reaction Energies

We have recently introduced the tensor hypercontraction (THC) method for electronic structure, including MP2. Here, we present an algorithm for THC-MP2 that lowers the memory requirements as well as the prefactor while maintaining the formal quartic scaling that we demonstrated previously. We also describe a procedure to optimize quadrature grids used in grid-based least-squares (LS) THC-MP2. We apply this algorithm to generate grids for first-row atoms with less than 100 points/atom while incurring negligible errors in the computed energies. We benchmark the LS-THC-MP2 method using optimized grids for a wide variety of tests sets including conformational energies and reaction barriers in both the cc-pVDZ and cc-pVTZ basis sets. These tests demonstrate that the THC methodology is not limited to small basis sets and that it incurs negligible errors in both absolute and relative energies