Workshop on Computational Chemistry and Its Applications (CCA) Session 1

Time and Date: 11:20 - 13:00 on 10th June 2014

Room: Bluewater II

Chair: Ponnadurai Ramasami

112 Computer-aided design of stereocontrol agents for radical polymerization [abstract]
Abstract: Controlling the stereochemistry of a polymer is highly desirable as this can affect its physical properties, such as its crystallinity, melting point, solubility and mechanical strength. Stereoregular polymers are normally prepared using expensive transition metal catalysts, which typically require demanding reaction conditions, and extending stereochemical-control to free radical polymerization has been a long sought goal. For monomers containing carbonyl groups certain Lewis acids have been shown to be capable of manipulating the stereochemistry, presumably via coordination to the polymer and/or monomer side chains so as to constrain their relative orientations during the propagation step. However, specific mechanistic details have yet to be clarified, and the degree of stereocontrol remains poor. To address these problems, we have been using computational chemistry, supported by polymerization experiments, to study the mechanism of stereocontrol in a variety of free-radical polymerization processes, and to predict the effect of solvents and novel control agents on polymer tacticity. Interestingly we have discovered that many Lewis acids do selectively coordinate to the terminal and penultimate radical side chains in a manner that should, in principle, facilitate control. However, this coordination is driven by the resulting stabilization of the propagating radical, which, ironically, deactivates it toward propagation. At the same time a less energetically favourable, non-controlling coordination mode involving the monomer side chain catalyzes propagation reaction and this provides the dominant reaction path. On this basis we suggest that simultaneous coordination to the monomer and propagating radical using bridging ligands or mixed Lewis acids may provide the way forward.
Michelle Coote, Benjamin Noble and Leesa Smith
38 Correlation between Franck-Condon Factors and Average Internulcear Separations for Diatomics Using the Fourier Grid Hamiltonian Method [abstract]
Abstract: The Fourier Grid Hamiltonian (FGH) Method is used to compute the vibrational eigenvalues and eigenfunctions of bound states of diatomic molecules. For these computations, the Hulburt and Hirschfelder (HH) potential model for diatomics is used. These potential energy functions are used for constructing and diagonalizing the molecular Hamiltonians. The vibrational wave functions for the ground and the excited states are used to calculate the Franck-Condon factors (FCFs), r-Centroids and average internuclear separations which play a significant role in determining the intensity of the bands in electronic transitions. The results of FCFs and r-Centroids for diatomic molecules such as H2, N2, CO, I2 and HF using the FGH method are compared with other methods. The FGH method provides an efficient and accurate alternative to calculate FCFs and other parameters that depend on the vibrational wavefunctions of the ground and exited electronic states. The Franck-Condon profiles indicate a strong correlation between the values of Franck-Condon factors and the mean internuclear separations for the corresponding transitions.
Mayank Kumar Dixit, Abhishek Jain, Bhalachandra Laxmanrao Tembe
122 Using hyperheuristics to improve the determination of the kinetic constants of a chemical reaction in heterogeneous phase [abstract]
Abstract: The reaction in the human stomach when neutralizing acid with an antacid tablet is simulated and the evolution over time of the concentration of all chemical species present in the reaction medium is obtained. The values of the kinetic parameters of the chemical reaction can be determined by integrating the equation of the reaction rate. This is a classical optimization problem that can be approached with metaheuristic methods. The use of a parallel, parameterized scheme for metaheuristics facilitates the development of metaheuristics and their application. The unified scheme can also be used to implement hyperheuristics on top of parameterized metaheuristics, so selecting appropriate values for the metaheuristic parameters, and consequently the metaheuristic itself. The hyperheuristic approach provides satisfactory values for the metaheuristic parameters and, consequently, satisfactory metaheuristics for the problem of determining the kinetic constants.
José Matías Cutillas Lozano, Domingo Gimenez
267 Speeding up Monte Carlo Molecular Simulation by a Non-Conservative Early Rejection Scheme [abstract]
Abstract: Molecular simulation describes fluid systems in detailed fashion. In general, they are more accurate and representative than equations of state. However, they require much more computational efforts. Several techniques have been developed in order to speed up Monte Carlo (MC) molecular simulations while preserving their precision. In particular, early rejection schemes are capable of reducing computational cost by reaching the rejection decision for the undesired MC trials at early stages. In this work, the introduced scheme is based on the fact that the energy due to interaction between any couple of Lennard-Jones (LJ) sites cannot be lower than a certain minimum energy that can be easily computed. It is called “non-conservative” as it generates slightly different Markov chains than the ones generated by the conventional algorithms. Nonetheless, the numerical experiments conducted show that these modifications are not significant, and both the proposed and the conventional methods converge to the same ensemble averages. In this study, the non-conservative scheme is first introduced and then compared to the conservative and bond formation early rejection schemes. The method was tested for LJ particles in canonical ensemble at several thermodynamic conditions. Results showed a relation between the thermodynamic conditions and the percentage of the CPU time saved. In principle, more CPU time was saved at conditions with high rejection rates for the MC trials. The non-conservative early rejection scheme was successful in saving more than 45 % of the CPU time needed by the conventional algorithms in canonical ensemble. Finally, this work presents an efficient early rejection method to accelerate MC molecular simulations which is easily extendable to other ensembles and complex molecules.
Ahmad Kadoura, Amgad Salama and Shuyu Sun