The Diamine Cation Is Not a Chemical Example Where Density Functional Theory Fails

TL;DR

This paper challenges previous claims that density functional theory (DFT) fails for the DMP+ cation, showing that high-level CCSD(T) calculations indicate DFT performs well, and that the PZ-SIC correction used earlier is inconsistent with these results.

Contribution

The study demonstrates that DFT accurately models the DMP+ cation and identifies the PZ-SIC correction as an outlier, correcting prior misconceptions about DFT's reliability for this system.

Findings

DFT performs well for DMP+ cation according to CCSD(T) benchmarks.

PZ-SIC correction is inconsistent with high-level CCSD(T) results.

Previous claims of DFT failure for this system are challenged.

Abstract

In a recent communication, Weber and co-workers presented a surprising study on charge-localization effects in the N,N'-dimethylpiperazine (DMP+) diamine cation to provide a stringent test of density functional theory (DFT) methods. Within their study, the authors examined various DFT methods and concluded that "all DFT functionals commonly used today, including hybrid functionals with exact exchange, fail to predict a stable charge-localized state." This surprising conclusion is based on the authors' use of a self-interaction correction (namely, complex-valued Perdew-Zunger Self-Interaction Correction (PZ-SIC)) to DFT, which appears to give excellent agreement with experiment and other wavefunction-based benchmarks. Since the publication of this recent communication, the same DMP+ molecule has been cited in numerous subsequent studies as a prototypical example of the importance of self-interaction corrections for accurately calculating other chemical systems. In this correspondence, we have carried out new high-level CCSD(T) analyses on the DMP+ cation to show that DFT actually performs quite well for this system (in contrast to their conclusion that all DFT functionals fail), whereas the PZ-SIC approach used by Weber et al. is the outlier that is inconsistent with the high-level CCSD(T) (coupled-cluster with single and double excitations and perturbative triples) calculations. Our new findings and analysis for this system are briefly discussed in this correspondence.