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Methods for accurate energies

Accurate Potential Energy Curve for C2

Diatomic carbon, C2, is found in hydrocarbon flames, comets, and the interstellar medium. C2 has an interesting electronic structure, including several low-lying excited states, and a ground state which exhibits strong multiconfigurational character even at equilibrium. The potential energy curve (PEC) of C2 dissociation is being calculated using the correlation energy extrapolation by intrinsic scaling (CEEIS) method. CEEIS was developed by Ruedenberg and Bytautas to estimate the full configuration interaction energy. The method employs a linear extrapolation between a series of configuration interaction calculations with truncated virtual orbital spaces. CEEIS has already been used to obtain high accuracy correlation energies in several diatomic systems (F2, O2, B2). Additional corrections to the PEC (core-electron correlation, scalar relativistic effects, spin-orbit coupling) were applied to these systems to obtain near spectroscopic accuracy. Highly accurate PECs for the ground state, well as several excited singlet states, were calculated as well as the vibrational frequencies for the surfaces.



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Malonic acid tautomerism

Malonic acid tautomer in highly concentrated particles

Malonic acid is a common organic aerosol found in the atmosphere. It exists in two tautomers, a ketone and enol form. Malonic acid can react with other atmospheric substances and its reactivity may be linked to the more common tautomer in a highly concentrated deliquesced particles. Using DFT, B3LYP allows for the elucidation and confirmation of IR frequencies indicating a more abundant enol structure in the malonic acid particles. DFT is useful in this application due to the need for calculations on large systems of hundreds of atoms or more. To efficiently sample many structures on the potential energy surface (PES) and to find several low energy conformations, effective fragment potential (EFP) methods with simulated annealing in GAMESS are employed.

Malonic acid with 4 waters
Figure. Malonic acid, enol tautomer with four waters.


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Cellulose

Understanding cellulose bonding and hydrolysis
John Baluyut

One project uses B3LYP and two-body fragment molecular orbital (FMO-2) calculations of energies to simulatethe rotation of dihedral angles formed by free hydroxyl groups in glucose residues of a cellulose fragment, and the rotation of torsion angles involving glycosidic linkages between glucose residues in cellulose (psi and phi angles). The use of FMO-2 allows us to pinpoint specific interactions among the glucose residues in cellulose that give rise to critical points in the potential energy curves obtained from the changing cellulose geometries.

John Baluyut

In another project, the reaction mechanism of the acid-catalyzed hydrolysis of cellobiose, the repeating unit of cellulose, is explored. In this project, EFP1 (effective fragment potential) water molecules are used to form a first solvation shell around the cellobiose molecule. The polarizable continuum model (PCM) is used to form a second solvation shell.

ajitha_Fig1A
ajitha_Fig1B
Fig A. I-alpha -12chains -144 units) FMO-study
Fig B. alpha-233- Cellotetraose FMO study
ajitha_Fig1C
ajitha_Fig1D
Fig C. celluloseIII +h2o ( 2 ns-- MD-charmm Potential)
Fig D. 8CelA + nanocellaose (4ns MD Charmm27 potential)

A. Molecular origins of cellulose recalcitrance: The goal is to understand hydrogen bonded network in polymorphs of cellulose using complex electronic structure theory methods. First FMO studies of cellulose. (Fig. A)

B. Using model of cellulose-assembly: The goal is to obtain FMO based statistical weights for strengths of hydrogen bonded interaction (case study cellotetraose)--(Fig. B)

C. All atom, FMO and CGMD simulations of crystalline cellulosic materials with explicit water: chain dynamics(Fig. C)

D. Biohydrolysis of cellulose: cellulose-protein binding dynamics using the all atom MD and FMO method (Fig. D)