Coupled cluster and configuration interaction quantum calculations of infrared fundamental intensities

摘要

The fundamental infrared intensities of HF, H_2O, HCN, CH _3F, CH_4, C_2H_6, C_2H _2, and C_2H_4 have been calculated at the quadratic configuration interaction and coupled cluster levels using a wide variety of basis sets. Both these levels contemplated single and double excitations and most of the coupled cluster calculations included triple excitations. The basis sets used included the cc-pVXZ (X = D,T,Q,5) sets, with and without polarization orbitals, the aug-cc-pVTZ set, and the Sadlej Z2PolX, Z3PolX, and pVTZ basis sets The theoretical results depended more on basis set variation rather than changes between the CCSD(T) and QCISD levels, although the results from these electron correlation treatment levels are quite different from those obtained at the second-order Moller-Plesset perturbation level. Principal component and hierarchical clustering analyses confirm the stronger basis set dependence of the intensity values. The cc-pVTZ basis set gives results that are about twice as accurate as those obtained using the smaller cc-pVDZ. However, basis sets that are larger than cc-pVTZ are not seen to improve accuracy. Most accurate intensity results were obtained using the CCSD(T)/cc-pVTZ and QCISD/cc-pVTZ levels with root mean square (rms) errors of 5.4 and 5.9 km mol ~(-1). This compares with a rms error of 11.7 km mol _(-1) for results obtained at the MP2/6-311++G(3d,3p) level that has often been used to calculate infrared fundamental intensities.