Benchmarking the Performance of Different Density Functionals for Quantum Anharmonic Vibrational Spectral Calculations.

Reliable interpretation of modern vibrational spectra requires the explicit inclusion of anharmonic effects, whose accuracy depends critically on the quality of the underlying potential energy surface (PES) determined by electronic structure theories (ESTs). Balancing accuracy and computational efficiency, particularly for large molecules, makes the choice of EST critical. Given the wide range of available density functionals and their varying performance, selecting an appropriate functional is therefore essential. In this work, we present a systematic assessment of various density functionals for constructing quantum anharmonic PESs within the N-mode representation by computing vibrational spectra for a diverse set of molecules using the VSCF and VSCF-PT2 methods. The accuracy and convergence of the calculated transition energies and intensities are evaluated against corresponding CCSD(T) results and experimental data. Overall, 61 density functionals with two electronic basis sets are benchmarked for more than 100 fundamentals, 56 overtone and combination bands, and 42 transition intensities. B2PLYP, B98, B97-2, B97-1, and B3PW91 emerge as the best-performing functionals, with mean absolute deviations of ∼20 and ∼30 cm- 1 relative to CCSD(T) and experiment, respectively. Practical recommendations are provided for selecting density functionals as black-box methods based on accuracy, computational cost, error estimation, and applicability to diverse molecular systems.