High level ab initio quantum mechanical predictions of infrared intensities

Vibrational intensities associated with the infrared spectra of H(2)O, C(2)H(2), HCN, H(2)CO, CH(4), and SiH(4) were theoretically predicted by applying ab initio quantum mechanical methods at seven different levels of theory. The self-consistent field, second-order Moller-Plesset perturbation method, configuration interaction with sin-le and double excitations, double excitations coupled-cluster, quadratic configuration interaction including single and double excitations (QCISD), coupled-cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory were applied. The atomic orbital basis sets employed included frozen-core double-zeta (DZ), triple-zeta (TZ), DZ plus single polarization (DZP), TZ plus double polarization (TZ2P), TZ plus triple polarization (TZ3P), TZ2P augmented with one set of higher angular momentum functions [TZ2P(f,d)], TZ2P(f,d) with one set of diffuse functions [TZ2P(f,d)+diff], TZ3P augmented with two sets of higher angular momentum functions [TZ3P(2f,2d)], [TZ3P(2f,2d)] with two sets of diffuse functions [TZ3P(2f,2d)+2diff], split valence plus polarization [6-311G(d,p) and 6-311G(3d,3p)], split valence with added polarization and diffuse functions [6-311++G(d,p) and 6-311++G(3d,3p)], Dunning's correlation consistent polarized valence [cc-pVXZ (X = 2-5)] basis sets, as well as augmented correlation consistent polarized valence [aug-cc-pVXZ (X = 2-5)] basis sets. The theoretical infrared intensities predicted at the different levels of theory for the studied molecules were compared with available experimental data. To complete the analysis, the predicted equilibrium geometries, dipole moments and harmonic vibrational frequencies were also compared with experiment. Several highly correlated types of wave functions employing extended basis sets produce intensity values in very good agreement with experiment. The best overall agreement between theory and experiment for all properties studied was obtained from highly correlated wave functions [QCISD, CCSD, CCSD(T)] combined with Dunning's correlation consistent aug-cc-pVXZ (X = 3-5) basis sets.