Date of Award

1-1-2011

Document Type

Campus Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

Chemistry

First Advisor

Vitaly A Rassolov

Second Advisor

Mark A Berg

Abstract

It is shown that both the delta-function operator and the RC operator provide comparable spin densities with extended basis sets ext-6-311++G** and ext-6-311++G(2d,2p). An optimum range parameter 0.25 or 0.35 a0 is appropriate for the RC operator with the extended basis sets. For standard basis sets 6-311++G** and 6-311++G(2d,2p), the delta function operator may yield considerable errors for heavy atoms due to the deficiency of the basis sets near the nuclei. To a large extent, however, this deficiency can be cured with the RC operator. With the same range parameter 0.25 or 0.35 a0 previously recommended, the errors are significantly reduced with respect to the delta function method. If a relatively larger range parameter 0.45 a0 is used, a further improvement can be obtained and the spin densities at the UB3LYP/6-311++G** or UB3LYP/6-311++G(2d,2p) level of methods are fairly identical to the ones at the UB3LYP/ext-6-311++G** or UB3LYP/ext-6-311++G(2d,2p) level of methods. Generally the UB3LYP/6-311++G** and the UB3LYP/ext-6-311++G** methods are able to provide satisfactory spin densities. For some anions, the UB3LYP/6-311++G(2d,2p) and the UB3LYP/ext-6-311++G(2d,2p) methods are needed to achieve better agreement with the experiment. It is demonstrated that the B3LYP functional combined with the standard Pople-type basis sets 6-311++G(2d,2p) and 6-311++G(2d,2p), which are designed for conventional electronic structure calculations, serves as a a efficient and reliable method for the calculation of spin densities of large organic radicals when the Rassolov-Chipman operator is utilized, while the method based on delta-function operator may lead to much less reliable results. An optimum range parameter of 0.45 is recommended.

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