The Importance of Tight f Basis Functions for Heavy p‑Block Oxides and Halides: A Parallel With Tight d functions in the Second Row

It is well-known that both wave function ab initio and DFT calculations
on second-row compounds exhibit anomalously slow basis set convergence
unless the basis sets are augmented with additional “tight”
(high-exponent) d functions, as in the cc-pV­(n+d)­Z and aug-cc-pV­(n+d)­Z
basis sets. This has been rationalized as being necessary for a better
description of the low-lying 3d orbital, which as the oxidation state
increases sinks low enough to act as a back-donation acceptor from
chalcogen and halogen lone pairs. This prompts the question whether
a similar phenomenon exists for the isovalent compounds of the heavy
p-block. We show that for the fourth and fifth row, this is the case,
but this time for tight f functions enhancing the description of the
low-lying 4f and 5f Rydberg orbitals, respectively. In the third-row
heavy p block, the 4f orbitals are too far up, while the 4d orbitals
are adequately covered by the basis functions already present to describe
the 3d subvalence orbitals.