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During the last year, the following specific goals were completed:

-   small water-alcohol, alcohol-alcohol, and alcohol-hydrocarbon clusters (including methanol, ethanol, tert-butanol (TBA), methane, neo-pentane, and benzene) have been studied with EFP Monte-Carlo. Obtained geometries and interaction energies in these clusters were compared with those calculated by the second-order perturbation theory MP2 and symmetry-adapted perturbation theory (SAPT) methods. Based on these benchmarks, the EFP potentials for water, alcohols, and hydrocarbons were refined (for example, now we use smaller basis set to perform the distributed multipolar analysis for the electrostatic part of the EFP potential). Apart of improving the agreement between EFP and MP2/SAPT for small clusters, the new EFP potentials appeared to be much more accurate in description of the structural properties of bulk liquids.

-   new EFP potentials were gathered in the effective fragment library (which now includes the EFP parameters for common solvents, alcohols, hydrocarbons, and low-energy tautomers of nucleic acid base (adenine, guanine, thymine, cytosine and uracil). The library is available within the GAMESS and Q-CHEM distributions.

-   mixing processes in water-TBA solutions have been studied by EFP-MD. Structural characteristics of these solutions (at different concentrations of TBA) were compared to available neutron scattering data. There is a very reasonable agreement between the EFP and experimental radial distribution functions (RDF) in terms of both heights of the peaks and relative intensities of different components. We found that for these mixtures, EFP performs better than the GROMOS96 force field often used for simulations of alcohol solutions.

A manuscript describing the investigation of incomplete mixing in water-TBA solutions is currently prepared for publication in JPCB.  

-   to streamline the EFP-MD calculations, a series of scripts have been developed.

These include (i) a script converting a PDB geometry into effective fragments; (ii) a script preparing initial configuration of a system of EFP fragments for MD simulations; (iii) a script analyzing and visualizing snapshots along the EFP-MD trajectory. The scripts are freely available within the GAMESS distribution.

-   development of “flexible fragments” – an extension of the traditional EFP potentials to molecules with flexible degrees of freedom such as long hydrocarbon chains and polypeptides – has been initiated. Initial benchmarks on torsional barriers in ethane and propane (divided as CH3-CH3 and CH3-CH2-CH3, respectively) are encouraging. Testing of the code on an extensive set of polypeptides and hydrocarbons is underway.

Additionally, several more developments, significantly extending the applicability of the EFP method, have been conducted. These include development of QM/EFP interface of the EFP code with coupled-cluster, equation-of-motion, and configuration interaction singles suite of programs. Two papers presenting initial benchmarks on EOM-CCSD/EFP and CIS/EFP and describing the underlying formalisms were written:

-   L.V. Slipchenko, J. Phys. Chem. A, 114 (33), 8824-8830 (2010) ;

-   P. Arora, L.V. Slipchenko, S.P. Webb, A. Defusco, M.S. Gordon, J. Phys. Chem. A, 114 (25), 6742–6750 (2010).

Moreover, successful implementation of electrostatic screening in EFP inspired a development of similar electrostatic screening functions between fragments in the fragment molecular orbital (FMO) method, and the paper describing this work was published:

-   D.G. Fedorov, L.V. Slipchenko, K. Kitaura, J. Phys. Chem. A, 114 (33), 8742-8753 (2010)

Several more applications using EFP and demonstrating the capabilities of this method are underway (i.e., calculation of ionization energies of thymine in water, calculation of absorption spectra of para-nitroaniline chromophore in various solvents, modeling Raman spectra of water around hydrophobic solutes, etc.).

The specific goals for the next year are:

-   complete implementation of flexible fragments;

-   populate the fragment library with effective fragments for polymer and protein modeling;

-   explore strategies for improved performance of EFP (faster exchange-repulsion);

-   investigate mixing and phase-separation in hydrocarbon-alcohol-water mixtures at different concentrations (alcohols: methanol, ethanol, TBA; hydrocarbons: methane, neo-pentane).  

Research within this project has the following impact:

-   development of the EFP method and its application to chemical/biological processes in solvents and environments is the main direction of Dr. Slipchenko’s research. Particular tasks fulfilled within the project (preparing the EFP library with improved EFP potentials, development of “flexible” fragments and scripts for analyzing and streamlining EFP-MD simulations) will be extremely useful for many future EFP-related projects;

-   three papers have been published and several manuscripts are in preparation;

-   participating graduate students Frank Emmert and Mike Hands as well as undergraduate student Stephanie Thompson got expertise in running classical and EFP Monte-Carlo and molecular dynamics simulations, as well as in developing perl and python scripts for analyzing MD data. Graduate student Levi Haupert got expertise in developing GAMESS codes. Undergraduate student Joanna Flick got expertise in programming in C++ for development of scripts for converting file formats.  

-   Prof. Slipchenko presented results related to this project at several national and international conferences, including the Telluride workshop “Many-Body Interactions: From Quantum Mechanics to Force Fields”. Graduate student Mike Hands attended the Gordon research conference on “Water & Aqueous Solutions” where he presented a poster describing his work on water-TBA mixing. Graduate student Frank Emmert and undergraduate Stephanie Thompson presented their work on Ohio International Symposium on Molecular Spectroscopy.