Research

The focus of our research is on the development of theoretical and computational approaches targeting the electronic structure of extended systems, such as photosynthetic and fluorescent proteins, molecular solids, polymers, and bulk liquids. Specifically, we develop universal force fields, QM/MM (quantum mechanics/molecular mechanics), and fragmentation techniques. These methods are broadly applicable to all areas of science and engineering; the resulting computer codes are implemented in the Q-Chem and GAMESS electronic structure packages. We use the developed techniques to investigate fundamental aspects of non-covalent interactions and the effect of the environment on electronic structure. We also employ these tools to solve problems of biological and industrial relevance. We maintain active collaborations with experimental groups, which provide new exciting ideas and inspiration for theoretical developments.

• Theory and development
• Electronic excitations in the condensed phase
• Non-covalent interactions
• Exciton interactions and energy transfer
• Electronic structure of open-shell and diradical species

Theory and development

Calculations in the condensed phase still remain a major challenge to the theoretical community. The increased number of nuclear and electronic degrees of freedom makes accurate ab initio calculations on a condensed phase system unfeasible long before the system can approach the bulk. One general approach to this type of problem is to separate a system into two parts, such that one (active) part is treated by quantum mechanical (QM) techniques, and the other, usually larger, part is calculated by using classical (molecular) mechanics (MM).

Traditionally, the MM part in QM/MM is included through parameterized force fields. A drawback of such an approach is the dependence on fitted parameters for a chosen force field, such that different parameterizations may be optimal for different problems and the best parameters are often not well defined. In order to overcome this problem, we employ the Effective Fragment Potential (EFP) method for the MM part. In the EFP, each solvent molecule is represented by a model potential with a set of parameters determined from a preparatory ab initio calculation. The uniqueness of the EFP method is that all parameters are derived from first principles, i.e., the method is free of parameter fitting. Through its force field, the EFP fragments can interact with each other and with ab initio components. Using the EFP method results in an accurate and first-principles-based description of the MM part and couplings between the QM and MM subsystems. Our EFP and QM/EFP methods are available in the GAMESS and Q-Chem electronic structure packages.

Development of the EFP method

Review of the general EFP method
Short-range damping functions in EFP: exponential-based and overlap-based
Coarse-graining of EFP
EFP implementation in Q-Chem
QM/EFP dispersion
EFP library project

QM/EFP schemes

EOM-CC/EFP
CIS/EFP
CIS(D)/EFP

FMO projects

We also develop the fragment molecular orbital (FMO) method, in collaboration with Dmitry Fedorov and Kazuo Kitaura (AIST, Japan). FMO is a fully quantum fragmentation technique in which one performs fragment calculations in the electrostatic field of other fragments, mutually self-consistent with each other.

Damping functions in FMO
Reviews of fragmentation techniques in 2009 and 2011

Electronic excitations in the condensed phase

Singlet and triplet states of para-nitroaniline solvated in water (left) and dioxane (right) and energy relaxation channels: internal conversion (IC) and intersystem crossing (ISC).

The environment alters chemical processes by perturbing electronic wave functions, changing relative energetics of polar and non-polar states/species, affecting couplings between electronic states, and by confining effects (caging). In extreme cases, a solvent may completely change the character of the electronic states of a solute and create new states that do not exist in the gas phase (e.g., so called charge-transfer-to-solvent states). Our goal is to understand how the environment affects electronic states and interactions (couplings) between them and to connect computational findings with spectroscopic observables. A series of excited-state QM/MM techniques in which the MM part is described by EFP that we developed allow us to pursue this goal.

Solvatochromic shifts in para-nitroaniline
Ionization energy in hydrated thymine
Review of QM/EFP for excited states

Non-covalent interactions

Non-covalent interactions govern the structure and functions of biological macromolecules such as DNA and proteins, the properties of condensed phase species like liquids and colloids, and adsorption processes of molecules on surfaces and interfaces. We investigate the nature of non-covalent interactions in molecular clusters, bulk liquids, and polymers. In many systems we consider the non-covalent interactions of different types (e.g., electrostatic and dispersive forces) compete with each other. While these situations are challenging for theory and require a balanced description of different non-covalent forces, they provide a rich variety of structural and bonding patterns that exhibit themselves in complicated spectroscopic observables.

Intermolecular interactions in small clusters

Substituted benzene dimers
Water-benzene clusters
Benzene and pyridine dimers
Interactions in DNA bases
EFP benchmark on S22 and S66 datasets

Intermolecular interactions in liquids

pi-H bonding in solvated benzene
Mixing in water-tert-butanol solutions
Affinity of halides to molecular hydrophobic interface

Intra-molecular interactions in biological polymers

Di- and tri- gamma-peptides

Exciton interactions and energy transfer

We investigate excitation energy transfer and photo-protection in the photosynthetic apparatus of plants. Understanding light-harvesting processes on the molecular level is a prerequisite for engineering photovoltaic devices and solar cells. Moreover, the underlying physical and chemical processes of energy transfer and dissipation are of a fundamental importance. Theoretical description of energy transfer in photosystems requires two components, that is: (i) the ability to accurately describe the electronic structure of photosynthetic pigments, chlorophylls and carotenoids; and (ii) the ability to model electronic and vibronic couplings that enable efficient energy transfer between the pigments. The excited-state QM/EFP and FMO methods address the former. To target the second aspect, we develop vibronic models of the Fulton-Gouterman type. Our vibronic model can treat multiple inter- and intra-monomer vibrational modes in asymmetric bi-chromophore and multi-chromophore systems.

Efficiency and rates of energy transfer in photosystems, such as in this peridinin-chlorophyll-protein light-harvesting complex in dinoflagellates, are controlled by the protein environment. Geometries and relative orientations of chromophores, chlorophylls and carotenoids, are determined by the shapes of protein cavities, whereas the character of electronically excited states of the chromophores is affected by non-uniform electric fields due to the protein. Details of the environment effects on the energy transfer and photoprotection in photosynthetic systems are not yet fully elucidated, as explicit inclusion of the protein in computational models is challenging. Consequently, the efficiency of artificial photosynthetic devices and solar cells is still far behind the efficiencies of natural photosystems, and the construction of modern photovoltaic devices is often done through trial and error.

Fulton-Gouterman model
Vibronic interactions in diphenylmethane

Electronic structure of open-shell and diradical species

Formation of bound charge-transfer state in ammonia-oxygen complex

We continue unraveling the mysteries of species with unusual electronic structure such as radicals and diradicals. In these projects, we employ state-of-the-art electronic structure methods of the equation-of-motion coupled cluster (EOM-CC) family, such as Spin-Flip (SF) and ionized potential (IP) methods.

Ammonia-oxygen exciplex
Singlet-triplet gaps in ethynyl-substituted cyclo-butadiene