A CHALLENGE FOR DENSITY-FUNCTIONAL THEORY - THE XONO AND XNO2 (X=F, CL, AND BR) MOLECULES

The equilibrium geometries, harmonic frequencies, dipole moments, infr ared intensities, and relative energies of the cis-XONO, trans-XONO, a nd XNO2 (X=F, Cl, and Br) have been investigated using four functional s in common use in Kohn-Sham density functional theory (DFT) calculati ons. Two of the functionals include non-local or gradient correction t erms, while the other two also incorporate some exact Hartree-Fock exc hange and are labeled hybrid functionals. The quality of the results o btained from the functionals is determined by comparison to previously published high-level coupled-cluster calculations. The hybrid functio nals perform better for prediction of the equilibrium geometries, wher e the two gradient corrected functionals yield qualitatively incorrect molecular structures for cis-FONO and cis-ClONO. None of the function als perform well in predicting all six harmonic frequencies, showing t hat the correlation between equilibrium geometries and harmonic freque ncies is not as strong for these DFT methods as it is for conventional wavefunction ab initio methods, such as coupled-cluster theory. Resul ts from the various functionals generally come into better agreement w ith each other and also with the coupled-cluster results moving down t he periodic table.