THE BALANCE BETWEEN THEORETICAL METHOD AND BASIS SET QUALITY - A SYSTEMATIC STUDY OF EQUILIBRIUM GEOMETRIES, DIPOLE-MOMENTS, HARMONIC VIBRATIONAL FREQUENCIES, AND INFRARED INTENSITIES

Analytic gradient methods have been used to predict the equilibrium ge ometries, dipole moments, harmonic vibrational frequencies, and infrar ed (IR) intensities of HCN, HNC, CO2, CH4, NH4+, HCCH, H2O, H2CO, NH3, and FCCH at the self-consistent-field (SCF), the single and double ex citations configuration interaction (CISD), the single and double exci tations coupled-cluster (CCSD), and the single, double, and perturbati ve triple excitations coupled-cluster [CCSD(T)] levels of theory. All studies were performed using a triple zeta plus double polarization (T Z2P) basis set and a TZ2P basis set augmented with one set of higher a ngular momentum functions [TZ (2df,2pd)]. The predicted equilibrium ge ometries, dipole moments, harmonic vibrational frequencies, and IR int ensities were compared to available experimental values. The geometrie s were predicted accurately at the highest levels of theory. Most of t he dipole moments were found to agree favorably with experiment. With the TZ2P basis set, the average absolute errors in harmonic vibrationa l 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 t he TZ(2df,2pd) basis set, the four methodologies yielded average absol ute errors of 10.3%, 6.3%, 3.7%, and 2.2%, respectively. When the abso lute errors for bending modes of triply bonded molecules and the al um brella mode of NH3 were excluded from the previous two sets of average s, 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 int ensities were generally close to given experimental values. Among the eight methodologies investigated in this research, the TZ2P CCSD and t he TZ(2df,2pd) CCSD(T) methods exhibited the best balance between theo retical method and basis set quality. This ''balance'' was evident in the simultaneous prediction of the most accurate values overall for th e molecular properties compared.