Energy derivatives in quasi-relativistic approaches

In chemical applications one is more interested in energy differences rather than in total energies. Of particular importance are derivatives of the energy with respect to some (external) parameter. This parameter might for example be one of the components of a homoge- 

The important point is now, that if one changes the external potential 

It is neither intended nor possible to give a comprehensive review of relativistic density functional calculations on small molecules. To be able to compare the methods described in Sec. 2, we will primarily discuss molecules for which computational results from a variety of different methods are aveulable. Molecules containing heavy elements from the left half of the periodic table (including lanthanides and actinides) will not be discussed, as most of this is covered in the eirti-cle by V. Pershina in this volume. Likewise, only calculations of molecular spectroscopic constants such as bond lengths (r ), (harmonic) vibrational frequencies ((0 ) and binding energies (D ) are 

A comparison of results from different relativistic density functional calculations gives some information on whether quasirelativistic treatments are good enough to describe the system under consideration. To assess the accuracy of the density functional results on the other hand requires comparison with experimental of high-level ab initio data. Only results from coupled cluster or Cl-type ab initio calculations are reported here. Although these methods are among the most accurate quantum chemical approaches, there is some variance in the quality of the results reported in the literature. This becomes evident by the substantial scatter sometimes shown by the ab initio results. Within this chapter, the quality of the ab initio results cannot be discussed extensively. We will just point out two possible sources of error  

Density functional results (BP functional unless otherwise noted)  

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