More Like this

1. Density Sensitivity of Empirical Functionals

   https://doi.org/doi.10.1021/acs.jpclett.0c03545  

   Song, Suhwan; Vuckovic, Stefan; Sim, Eunji; and Burke, Kieron (January 2021, The journal of physical chemistry letters)

   Empirical fitting of parameters in approximate density functionals is common. Such fits conflate errors in the self-consistent density with errors in the energy functional, but density-corrected DFT (DC-DFT) separates these two. We illustrate with catastrophic failures of a toy functional applied to H2+ at varying bond lengths, where the standard fitting procedure misses the exact functional; Grimme’s D3 fit to noncovalent interactions, which can be contaminated by large density errors such as in the WATER27 and B30 data sets; and double-hybrids trained on self-consistent densities, which can perform poorly on systems with density-driven errors. In these cases, more accurate results are found at no additional cost by using Hartree–Fock (HF) densities instead of self-consistent densities. For binding energies of small water clusters, errors are greatly reduced. Range-separated hybrids with 100% HF at large distances suffer much less from this effect. 

   more » « less
2. Hartree–Fock density functional theory works through error cancellation for the interaction energies of halogen and chalcogen bonded complexes

   https://doi.org/10.1063/5.0289571  

   Pangeni, Niraj; Shahi, Chandra; Perdew, John P; Subramanian, Vishal; Kanungo, Bikash; Gavini, Vikram; Ruzsinszky, Adrienn (February 2026, The Journal of Chemical Physics)

   Unusually large energy errors of semi-local density functional approximations (DFAs) for molecules are often strongly reduced by using the Hartree–Fock (HF) electron density instead of the self-consistent DFA density. For reaction barriers and water clusters, some of us earlier found that HF-density functional theory (DFT) succeeds not because the HF density is accurate but due to the cancellation of negative functional-driven error (FE) by positive density-driven error (DE). Since DE, as defined here, is biased toward the self-consistent DFA density and against the HF density, the DE of the HF density is referred to as non-variational density over-localization (NVDO). In this work, we show that interaction energy errors in halogen- and chalcogen-bonded complexes in the B30 dataset are not dominated by density-driven error. Instead, HF-DFT again succeeds through FE–NVDO cancellation. Benchmark Kohn–Sham inversions of coupled-cluster densities for NH3⋯ClF, Cl−⋯ClF, Cl−⋯SF2, Cl−⋯SCF2, and Cl−⋯PF3 provide strong evidence for this cancellation. For additional complexes, we employ the long-range-corrected hybrid LCωPBE as a proxy for electron-transfer errors in the exact density. We also examine several self-interaction correction (SIC) methods and find significant improvement from FLOSIC. We point out common features of the density errors in the NH3⋯ClF and Cl−⋯ClF complexes and three transition states, arguing that significant density-driven errors of energy arise only from electron-transfer errors. We also highlight a common feature in our present and previous work: long bonds can lead to non-negligible functional-driven self-interaction error of the energy from otherwise accurate semi-local functionals in transition states, water clusters, and halogen or chalcogen bonds. 

   more » « less
3. Measuring Density-Driven Errors Using Kohn–Sham Inversion

   https://doi.org/doi.org/10.1021/acs.jctc.0c00391  

   Nam, Seungsoo; Song, Suhwan; Sim, Eunji; and Burke, Kieron (July 2020, Journal of chemical theory and computation)

   Kohn–Sham (KS) inversion, that is, the finding of the exact KS potential for a given density, is difficult in localized basis sets. We study the precision and reliability of several inversion schemes, finding estimates of density-driven errors at a useful level of accuracy. In typical cases of substantial density-driven errors, Hartree–Fock density functional theory (HF-DFT) is almost as accurate as DFT evaluated on CCSD(T) densities. A simple approximation in practical HF-DFT also makes errors much smaller than the density-driven errors being calculated. Two paradigm examples, stretched NaCl and the HO·Cl– radical, illustrate just how accurate HF-DFT is. 

   more » « less
4. Examining the Impact of Local Constraint Violations on Energy Computations in DFT

   https://doi.org/10.1002/jcc.70005  

   Khanna, Vaibhav; Kanungo, Bikash; Gavini, Vikram; Tewari, Ambuj; Zimmerman, Paul M (January 2025, Journal of Computational Chemistry)

   This work examines the impact of locally imposed constraints in Density Functional Theory (DFT). Using a metric referred to as the extent of violation index (EVI), we quantify how well exchange‐correlation functionals adhere to local constraints. Applying EVIs to a diverse set of molecules for GGA functionals reveals constraint violations, particularly for semi‐empirical functionals. We leverage EVIs to explore potential connections between these violations and errors in chemical properties. While no correlation is observed for atomization energies, a significant statistical correlation emerges between EVIs and total energies. Similarly, the analysis of reaction energies suggests weak positive correlations for specific constraints. However, definitive conclusions about error cancellation mechanisms cannot be made at this time. These observations revealed by EVIs may be useful for consideration when designing future generations of semilocal functionals. 

   more » « less
5. Implementation of symmetry-adapted perturbation theory based on density functional theory and using hybrid exchange–correlation kernels for dispersion terms

   https://doi.org/10.1063/5.0090688  

   Xie, Yi; Smith, Daniel G.; Sherrill, C. David (July 2022, The Journal of Chemical Physics)

   We report the implementation of a symmetry-adapted perturbation theory algorithm based on a density functional theory [SAPT(DFT)] description of monomers. The implementation adopts a density-fitting treatment of hybrid exchange–correlation kernels to enable the description of monomers with hybrid functionals, as in the algorithm by Bukowski, Podeszwa, and Szalewicz [Chem. Phys. Lett. 414, 111 (2005)]. We have improved the algorithm by increasing numerical stability with QR factorization and optimized the computation of the exchange–correlation kernel with its 2-index density-fitted representation. The algorithm scales as O( N 5 ) formally and is usable for systems with up to ∼3000 basis functions, as demonstrated for the C 60 –buckycatcher complex with the aug-cc-pVDZ basis set. The hybrid-kernel-based SAPT(DFT) algorithm is shown to be as accurate as SAPT(DFT) implementations based on local effective exact exchange potentials obtained from the local Hartree–Fock (LHF) method while avoiding the lower-scaling [ O( N 4 )] but iterative and sometimes hard-to-converge LHF process. The hybrid-kernel algorithm outperforms Hartree–Fock-based SAPT (SAPT0) for the S66 test set, and its accuracy is comparable to the many-body perturbation theory based SAPT2+ approach, which scales as O( N 7 ), although SAPT2+ exhibits a more narrow distribution of errors. 

   more » « less