Constructing A Library of Potential Energy Functions from Coupled Cluster Calculations to Be Used in the SCC-DFTB Parameterization

• Main Authors: Yang, Po-Yu, 楊博宇
• Other Authors: Witek, Henryk
• Format: Others
• Language: en_US
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/32219709298064952768

碩士 === 國立交通大學 === 應用化學系分子科學碩博士班 === 99 === The SCC-DFTB method is a powerful semi-empirical method of quantum chemistry, which is able to treat huge molecular systems. However, SCC-DFTB has certain limitations, which diminish its strength for particular chemical applications. The first limitation concerns the fact that accurate energies, equilibrium geometries, and vibrational frequencies cannot be obtained with the existing parameter sets simultaneously. In the current parameterization of SCC-DFTB, insufficient number of reference potential energy surfaces were used for determination of the parameters. The second limitation comes from the fact that the current SCC-DFTB parameter sets are available only for few selected elements. To overcome those limitations, numerous reference potential energy surfaces representing various bonding mechanisms between atoms are required in the parameterization process. For generating reference potential energy surfaces usable in a fast parameterization process, most of high-level accurate quantum chemical methods are too expensive to be employed. Therefore, we construct a collection of force-field like potential energy functions on the 4-th order Taylor approximation to expand those functions. Generated by accurate Coupled cluster calculations, our potential energy function not only provide us with accurate potential energy surfaces but also allow for substantial savings in the computational time. The final library of potential energy functions were determined for 74 common molecules containing the elements of the first, second, and third row of the periodic table of elements. The list of these molecules attempts to represent the most typical bonding situations between these elements. We conclude the derivation of the library of potential energy functions by presenting a verification algorithm designed to validate the accuracy of our library. In the fitting process, we are able to control the RMS error to be less than 10-4 a.u. in all the studied cases. We have also implemented each of potential energy function as an external program, which can be invoked from the Gaussian09 program, for performing geometry optimization and calculating vibrational frequencies. In all of studied cases, our library of potential energy functions can reproduce the equilibrium geometry and vibrational frequencies giving results almost identical with those from the CCSD(T)/cc-pVTZ calculation. For all the calculated vibrational frequencies, the error to CCSD(T) is smaller than 10 cm-1.