|Example #1 (Larger view)|
|Example #2 (Larger view)|
1) Hybrid Methods for Large Chromophores in Complex Environments
|Example (Larger view)|
Chromphores of interest in real applications are often too large to be treated entirely at a high level of theory. Therefore, hybrid methods that combine a high quantum mechanical (QM) level of theory for the core region, and lower levels for outer layers are necessary to strike the right balance between computational cost and accuracy. We develop hybrid methods that combine accurate QM methods (e.g., from coupled cluster theory) with cheaper QM methods (e.g., from density functional theory) and with polarizable classical models (e.g., implicit solvation models or polarizable force fields) to describe the electronic response properties of large chromophores in complex environments.
When molecules are too large to be treated entirely at an accurate QM level, hybrid methods are often employed to reduce the computational cost. In hybrid methods, the system is separated in layers where the most important region is treated with a high level of theory while the rest is treated at a lower, more manageable level of theory. The success of this approach is often connected with the ability to treat the interaction between the layers appropriately. This becomes even more important when we want to study electronic excited states because of the more polarizable nature of the electron density.
We are working on developing methods to account for mutual polarization between high and low levels of theory in a self-consistent manner, see scheme in Figure. Starting from the ONIOM (Our own N-layer molecular Orbital molecular Mechanics) method of Morokuma for excitation energies, we are working on defining polarizable embedding fields whose parameters are computed on-the-fly from the low-level calculation. Additionally, we are developing techniques to extrapolate multiple excited states simultaneously to avoid the problem of state-matching between different levels of theory.
We also develop strategies to couple CC methods with these solvation models, aiming at the best compromise between accuracy and computational effort. We consider both implicit and explicit polarizable solvation models, shown in Figure, for ground and excited state molecular properties. Our goal is to take advantage of the high-quality of the CC results with the computational efficiency of these solvation models to simulate the spectra of complex chromophores in bulk solution or at the interface between solid and liquid phase.
S. Ren, F. Lipparini, B. Mennucci, M. Caricato*, Coupled Cluster Theory with Induced Dipole Polarizable Embedding for Ground and Excited States; J. Chem. Theory Comput., 15, (2019) 4485.
M. Caricato*, CCSD-PCM Excited State Energy Gradients with the Linear Response Singles Approximation to Study the Photochemistry of Molecules in Solution; ChemPhotoChem, 3, (2019) 1.
M. Caricato*, Linear response coupled cluster theory with the polarizable continuum model within the singles approximation for the solvent response; J. Chem. Phys., 148, (2018) 134113.
S. Ren, J. Harms, M. Caricato*, An EOM-CCSD-PCM Benchmark for Electronic Excitation Energies of Solvated Molecules; J. Chem. Theory Comput., 13, (2017) 117.
A. Biancardi, J. Barnes, M. Caricato*, Point charge embedding for ONIOM excited states calculations; J. Chem. Phys., 145, (2016) 224109.
S. Ren, M. Caricato*, Multi-state extrapolation of UV/Vis absorption spectra with QM/QM hybrid methods; J. Chem. Phys., 144, (2016) 184102.
2) Chemical Intuition in Chiroptical Spectroscopy
|Example (Larger view)|
Despite a long history of use to probe chiral systems, it remains poorly understood how the observed optical activity of a molecule correlates with its structure. We are working to unravel this relationship by developing new electronic structure methods that can aid in characterizing the specific interactions that lead to chiroptical phenomena.
The chirality, or handedness, of a molecule can have a profound effect on its properties; a drug that is beneficial in one form may have a mirror image that is toxic. Similarly, a catalyst might react readily with one handedness of a molecule and not at all with the other. Optical activity, the rotation of plane polarized light after passing through a chiral medium, has been an invaluable tool for studying such compounds for more than two centuries. While this technique is widely used, it remains chemically unintuitive how the magnitude, or even the direction, of optical rotation (OR) is linked to the structure of a molecule. We are elucidating this structure-property relationship by developing and applying new electronic structure methods to study chiroptical phenomena.
A crucial open problem in this area is how to properly model the effect of solvation; most experimental measurements of OR are done in solution, but it is too costly to simulate the surrounding solvent molecules quantum mechanically. The solvent shell can have chirality induced by the solute, causing additional OR that goes unaccounted for in simulations of the solute alone. We are developing tools to simplify the treatment of solvated systems and assign OR contributions to specific functional groups in order to better distinguish between molecular and solvent effects. Another approach to attack this problem is developing methods to calculate OR of solid systems. While the OR in solution is the same in each direction due to isotropic averaging, solids give a different response along each crystal axis, making it easier to disentangle what chemical features are producing the OR. Calculating OR for solids, in particular molecular crystals, could make clearer the distinction between OR caused by molecular versus environmental/supermolecular chirality. We are also interested in isotopically chiral molecules. Seeding a chiral superstructure with isotopically chiral monomers can cause it to favor a particular handedness; we are analyzing if the OR of these monomers hints at the extent to which a given handedness is preferred and whether the OR can be tuned through site specific isotopic substitution.
T. Balduf, M. Caricato*, Helical Chains of Diatomic Molecules as a Model for Solid State Optical Rotation; J. Phys. Chem. C, 123, (2019) 4329.
T. Aharon, M. Caricato*, Configuration Space Analysis of the Specific Rotation of Helicenes; J. Phys. Chem. A, 123, (2019) 4406.
T. Aharon, P. Lemler, P. H. Vaccaro*, M. Caricato*, Comparison of Measured and Predicted Specific Optical Rotation in Gas and Solution Phases: A Test for the Polarizable Continuum Model of Solvation; Chirality, 30, (2018) 38
M. Caricato*, Orbital Analysis of Molecular Optical Activity Based on Configuration Rotatory Strength; J. Chem. Theory Comput., 11, (2015) 1349.
3) Ground and Excited State Raman Spectroscopy of Conjugated Thiophenes Oligomers
|Example (Larger view)|
This project is in collaboration with the Elles group. We are studying the electronic structure of an interesting class of prototype molecules: phenyl-thiophenes (PTs). PTs are model systems for more complicated thiophene polymers that have many interesting applications in materials science. We are interested in studying how absorption of light affects the excited state dynamics both from the experimental and theoretical points of view. We are therefore evaluating ground and excited state singlet and triplet potential energy surfaces, and simulating IR and Raman spectra on all surfaces, see Figure for diphenyl-thiophene. The information gained on these molecules will be then extended to more complicated compounds in solution and in solid phase.
M. S. Barclay, M. Caricato*, C. G. Elles*, Femtosecond Stimulated Raman Scattering from Triplet Electronic States: Experimental and Theoretical Study of Resonance Enhancements; J. Phys. Chem. A, 123, (2019) 7720.
T. J. Quincy, M. S. Barclay, M. Caricato*, C. G. Elles*, Probing Dynamics in Higher-Lying Electronic States with Resonance-Enhanced Femtosecond Stimulated Raman Spectroscopy; J. Phys. Chem. A, 122, (2018) 8308.
M. S. Barclay, T. J. Quincy, D. B. Williams-Young, M. Caricato*, C. G. Elles*, Accurate Assignments of Excited-State Resonance Raman Spectra: A Benchmark Study Combining Experiment and Theory; J. Phys. Chem. A, 121, (2017) 7937.
4) Charge Transfer via Electronic Coupling
|Example (Larger view)|
Charge transfer processes are ubiquitous across chemistry, but they are computationally intensive to simulate. We are developing approaches to efficiently and accurately simulate charge transfer using diabatic electronic couplings.
From cellular respiration and photosynthesis to the design of molecular devices and electrocatalysts, understanding and controlling charge transfer is important across a wide range of chemical disciplines. In recent years, simulations of charge transfer have become commonplace in determining the mechanism for a given process or finding what structural parameters can be tuned in order to direct and enhance current flow. Simulation can offer insights that are not readily obtainable from experiment, providing a molecular level view of a process and the means to study as yet unsynthesized compounds. A key challenge in this area is that most systems of practical interest (e.g. proteins, amorphous clusters, nanoscale devices) are too large to simulate fully quantum mechanically. We aim to address this problem by developing more computationally efficient methods to model charge transfer.
Fundamentally, charge transfer, either through-space or through-bonds, directs charge carriers from a donor to an acceptor. To model these processes, most approaches require expensive excited state calculations and multiple subcalculations of the donor and acceptor fragments individually. We have devised an algorithm to determine electronic couplings, which are directly related to charge transfer rates, from a single ground state DFT calculation. These same issues are exacerbated in simulations of solids, where the size of the systems require periodic boundary conditions (PBCs) to be used. More efficient methods would greatly aid efforts to optimize the performance of nanodevices bound to electrodes. We are currently working to extend our DFT coupling algorithm to use PBCs for cases where the donor and acceptor are chemically bound.
A. Biancardi, M. Caricato*, A Benchmark Study of Electronic Couplings in Donor–Bridge–Acceptor Systems with the FMR-B Method; J. Chem. Theory Comput., 14, (2018) 2007.
A. Biancardi, S. C. Martin, C. Liss, M. Caricato*, Electronic Coupling for Donor-Bridge-Acceptor Systems with a Bridge-Overlap Approach; J. Chem. Theory Comput., 13, (2017) 4154.
A. Biancardi, C. Caraiani, W.-L. Chan, M. Caricato*, How the Number of Layers and Relative Position Modulate the Interlayer Electron Transfer in π-Stacked 2D Materials; J. Phys. Chem. Lett., 8, (2017) 1365.
A. Biancardi, M. Caricato*, Evaluation of Electronic Coupling in Solids from Ab Initio Periodic Boundary Condition Calculations: The Case of Pentacene Crystal and Bilayer Graphene; J. Phys. Chem. C, 120, (2016) 17939.