The balance between theoretical method and basis set quality: A systematic study of equilibrium geometries, dipole moments, harmonic vibrational frequencies, and infrared intensities

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Abstract
Analytic gradient methods have been used to predict the equilibrium geometries, dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities of HCN, HNC, CO2, CH4, NH4+, HCCH, H2O, H2CO, NH3, and FCCH at the self‐consistent‐field (SCF), the single and double excitations configuration interaction (CISD), the single and double excitations coupled‐cluster (CCSD), and the single, double, and perturbative triple excitations coupled‐cluster [CCSD(T)] levels of theory. All studies were performed using a triple zeta plus double polarization (TZ2P) basis set and a TZ2P basis set augmented with one set of higher angular momentum functions [TZ (2df,2pd)]. The predicted equilibrium geometries, dipole moments, harmonic vibrational frequencies, and IR intensities were compared to available experimental values. The geometries were predicted accurately at the highest levels of theory. Most of the dipole moments were found to agree favorably with experiment. With the TZ2P basis set, the average absolute errors in harmonic vibrational frequencies with respect to experiment were 9.9%, 3.8%, 1.5%, and 2.3% for the SCF, CISD, CCSD, and CCSD(T) methods, respectively. With the TZ(2df,2pd) basis set, the four methodologies yielded average absolute errors of 10.3%, 6.3%, 3.7%, and 2.2%, respectively. When the absolute errors for bending modes of triply bonded molecules and the a1 umbrella mode of NH3 were excluded from the previous two sets of averages, the TZ2P average errors became 7.3% (SCF), 3.0% (CISD), 1.1% (CCSD), and 1.1% [CCSD(T)], and the TZ(2df,2pd) average errors became 7.4% (SCF), 3.5% (CISD), 1.5% (CCSD), and 0.6% [CCSD(T)]. Theoretical IR intensities were generally close to given experimental values. Among the eight methodologies investigated in this research, the TZ2P CCSD and the TZ(2df,2pd) CCSD(T) methods exhibited the best balance between theoretical method and basis set quality. This ‘‘balance’’ was evident in the simultaneous prediction of the most accurate values overall for the molecular properties compared.