Frozen core or all-electron?

In general we recommend the use of frozen core basis sets if available. Especially for the heavier atoms the number of functions is much smaller than for their all-electron counterparts. Our tests indicate that the error made by invoking the frozen core approximation is usually clearly smaller than the difference with respect to slightly higher quality basis sets. For the ZORA/QZ4P basis sets, only all-electron basis sets are available as these are intended for near basis set limit calculations only in which the CPU time is not a major concern.

Geometry optimizations involving atoms with a too large frozen core may give rise to numerical problems. In such cases it is recommendable to use a smaller frozen core. In previous occurrences we have removed such atomicdata files from the database. In the BAND basis sets such large frozen cores still exist in some cases. They should be handled with care. They have not been removed as they will only lead to problems in special cases and will reduce CPU times in many other calculations.

For accurate results on properties like nuclear magnetic dipole hyperfine interactions (ESR), nuclear quadrupole coupling constants, and chemical shifts (NMR), all electron basis sets are needed on the interesting atoms. For such properties tight functions might be necessary for high accuracy, especially in a ZORA calculation.