Frozen core calculations

Frozen core settings in ORCA

In post-HF calculations only the valence electrons are typically correlated. The other electrons are kept frozen and define a frozen core. Frozen cores differ between quantum chemistry codes and basis sets have been designed for specific frozen cores.

In ORCA 4.0, frozen core is always the default: Use !NoFrozencore to turn off

Note that going from a frozen core calculation to an all-electron (no frozen core) usually requires one to switch from a valence basis set to a basis set that has been designed for describing core-core and core-valence effects. For example one might have to switch from the cc-pVTZ basis set to a cc-pCVTZ or cc-pwCVTZ basis set.

See ORCA manual about including core orbitals in the correlation treatment in DLPNO-CC calculations.

In ORCA 4.0 the default frozen core definitions were changed for large parts of the periodic table: heavy elements mostly. An automatic frozencore checker was also implemented that reorders orbitals for situations where the assigned molecular frozen core (based on counting core orbitals) is wrong because valence orbitals on light atoms happen to be lower in energy than the core orbitals of heavy atoms.

See manual for more information about frozen core behaviour in ORCA.

Figure and table below shows the default frozen core since introduced in ORCA 4.