Accuracy of Energy Approximations of Proton Transfer Reactions of NH3(H2O)3 Clusters Using Fragment Based Methods

Winona, Minnesota

The discipline of computational chemistry often requires the calculation of the total energy in a system. Traditionally, conventional quantum methods have been used to determine the total energy of molecules which become computationally more expensive as the size of the system increases. Fragment-based methods are often used to determine the potential energy of large molecules. Fragment-based methods including many-body (MB), electrostatically embedded many-body (EE-MB), and fragment molecular orbital (FMO) were employed to determine the energies of the steps in a proton transfer reaction of a NH₃(H₂O)₃ cluster. The program GAMESS was used for the FMO energy calculations and NWChem was used for the MB and EE-MB energy calculations. It was found that the energy of the proton transfer was best approximated by the FMO-3 method paired with the 6-31G* basis set with an error of 0.0820 kcal/mol. The method that proved to be the least accurate was the PA paired with the 6-31G* basis set, with an average unsigned relative deviation of 10.808 kcal/mol. It was found that the 6-31G* basis set allows for greater accuracy in the FMO-2, FMO-3, 3B energy approximations. It was also determined that the use of the 6-311++G(2d,2p) basis set for the PA, EE-PA, and EE-3B methods reduced the error of the approximations.