Bonding, relativistic effects Correlation energy

The accuracy of the various relativistic, non-relativistic, correlated and non-correlated methods in comparison with experimental results is shown in Table 1 for AuH, a sort of a test molecule (see also [63,64]). The data of Table 1 demonstrate the importance of relativistic and electron correlation effects. Thus, relativistic effects diminish the equilibrium bond length (Re) by 0.26 A (the HF-DF difference without correlation) or by 0.21 A (the HF+MP2 - DF+MP2 difference with correlation), and enlarge the binding energy (De) by 0.70 eV (the HF-DF difference without correlation) or by 2.21 eV (the HF+MP2 - DF+MP2 difference with correlation). Correlation diminishes Re on the DF level by 0.07 eV, but enhances De by 1.34 eV. Thus, even for AuH correlation amounts almost to 50% of the chemical bond strength. No additivity of correlation and relativistic effects was shown. 

Table 1. Accuracy of different molecular methods for AuH showing importance of relativistic and correlation effects. RL. is the equilibrium bond length and Pg is the binding energy...

In light molecules, spin-orbit coupling predominantly affects spectral properties such as fine-structure and transition probabilities. In heavy-element compounds spin-mbit interaction is also of concern for bond distances and binding energies. Independent of the spin-orbit interaction scheme, it is indispensable to employ methods which take electron correlation and relativistic effects into account. Recent review articles give an overview of progress in this field (Hess and Marian 2000 Marian 2000). 

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