10 research outputs found
Mechanism for the Enhanced Excited-State Lewis Acidity of Methyl Viologen
Aqueous solutions of methyl viologen
(MV<sup>2+</sup>) exhibit
anomalous fluorescence behavior. Although it has long fluorescence
lifetimes in polar solvents such as acetonitrile, MV<sup>2+</sup> 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>, 2737â2748) 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, MV<sup>2+</sup> 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 MV<sup>2+</sup> are performed in order
to test the hypothesis that MV<sup>2+</sup> acts as a Lewis photoacid
and to elucidate a mechanism for this behavior. These simulations
show that the Lewis acidity of MV<sup>2+</sup> is indeed enhanced
by photoexcitation. On its S<sub>1</sub> excited state, MV<sup>2+</sup> 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 MV<sup>2+</sup> 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: 2â(2â˛-Hydroxy-5â˛-methylphenyl)Âbenzotriazole
A large
number of common photostabilizers are based on the 2-(2â˛-hydroxyphenyl)Âbenzotriazole
structure. One common example is 2-(2â˛-hydroxy-5â˛-methylphenyl)Âbenzotriazole,
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 FranckâCondon 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 â¨<i>SĚ</i><sup>2</sup>âŠ, 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 â¨<i>SĚ</i><sup>2</sup>âŠ. 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><sup><b>2</b></sup><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 LoĚwdin 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 (Ag<sub>19</sub>) 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
âBalancingâ the Block DavidsonâLiu Algorithm
We describe a simple modification
(âbalancingâ) of
the block DavidsonâLiu 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 âź2Ă
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 [JurecĚka, P.; SĚponer, J.; CĚernyĚ,
J.; Hobza, P. <i>Phys. Chem. Chem. Phys.</i> <b>2006</b>, <i>8</i>, 1985; RĚezaĚcĚ, 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 [CCSDÂ(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 Ď 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 Ă
), the electrostatic
contribution is nearly always attractive, providing further evidence
for the importance of charge-penetration effects in ĎâĎ
interactions (a term neglected in classical force fields). Comparison
of the computed stacking energies with those from model complexes
made of the âparentâ nucleobases purine and 2-pyrimidone
indicates that chemical substituents in DNA and RNA account for 20â40%
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 MøllerâPlesset 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