Relativistic methods
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The ZORA relativistic approach is often superior and in other cases at
least similar to the older Pauli method. In particular, for all-
electron calculations generally, and for very heavy elements even
within the frozen core approach, the Pauli method may exhibit
significant shortcomings. This is mostly due to the variational
instability of the Pauli formalism in the deep-core region near the
nucleus. The bigger the basis set and the smaller the frozen core, the
more likely this will show up, while generally speaking you might be
tempted to use smaller cores and bigger basis sets to *improve* your
results. The ZORA approach does not suffer from these problems and is,
therefore, highly recommended over the Pauli formalism.
