3Σ− and 3Π states of GeC and GeSi: The problematic dissociation energy of GeC

The 3Σ− and 3Π states for the GeC and GeSi diatomics have been investigated at the highly correlated coupled-cluster levels of theory. Large basis sets [including TZ3P(2d,2f)+2diff, cc-pVQZ, and aug-cc-pVQZ] were employed in order to predict reliable values for the experimentally unknown spectroscopic properties. The ground states were confirmed to be the 3Π state for GeC and the 3Σ− state for GeSi. Advanced theoretical treatments such as inclusion of core-valence correlation, scalar relativity, and complete basis set extrapolations have been performed to determine accurate energetic properties. The dissociation energies (D0) of 91.4 kcal/mol and 72.9 kcal/mol have been predicted for the X 3Π state of GeC and X 3Σ− state of GeSi, respectively. It is observed that the theoretical value of 72.9 kcal/mol for GeSi is in very good agreement with the experimental value of 71±5 kcal/mol. However, the predicted dissociation energy for the X 3Π state of GeC is found to be 18 kcal/mol smaller than the mass spectrom...