Applications of the coupled-cluster theory on the novel H, Si, C, and Ge containing small molecules : effects of scalar relativity
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The highly correlated ab initio coupled-cluster theories have been employed on the ground and rst excited electronic states of the HCSi and HCGe tri-atomic molecules, and on the ground electronic states of the GeC2, Si2H3, and Si2H4. The main reason for the choice of these molecules is that very little or even nothing is known about the structure, energetic, and other rst and second order molecular properties. Also, these molecules are of interest because of their potential applications in semiconductors and optoelectronics, in surface growth processes, and their possible existence in the circumstellar atmospheres of evolved carbon stars. Large basis sets (e.g. TZ3P(2f,2d)+2diff and cc-pVQZ) have been employed in conjunction with the very sophisticated quantum mechanical methods such as CCSD(T), CCSD(2), and CCSDT. Equation of motion coupled cluster theories were employed in order to determine some excited state properties, which cannot be determined using standard quantum mechanical methods due to possible variational collapses. Challenging problems such as characterization of the Renner-Teller splitting in the ground ~X 2 states of HCSi and HCGe, determination of the true ground state equilibrium geometry for the elusive GeC2, and searching the unanticipated mono- and di-bridged isomers of the Si2H3 and Si2H4 molecules have been focused. Investigation of the e ects of the scalar relativistic corrections on some molecular properties as well as on the Renner-Teller splitting has been another focus of interest throughout the study. In some cases, corrections due to zero-point vibrational energies (ZPVE) and core-valence interactions have been determined in order to predict reliable spectroscopic constants. The theoretical predictions for the HCSi and HCGe tri-atomic molecules were compared with the few existent experimental and theoretical works in literature. It has been observed that coupled cluster theory in conjunction with large basis sets is able to predict bond distances within 0.1 and energetic properties within 1 kcal/mol accuracy. Inclusion of the scalar relativistic corrections even produced better estimates. The equilibrium geometry for the GeC2 molecule in its ground state is predicted to be L-Shaped rather than T-shaped as in the case of SiC2. For the structural predictions on the Si2H3 and Si2H4 molecules, collaboration with the Harvard experimentalists showed excellent agreements.