A systematic theoretical study of harmonic vibrational frequencies: The single and double excitation coupled cluster (CCSD) method

Recently developed analytic CCSD gradient methods have been used to predict the harmonic vibrational frequencies of six molecules: CH4, NH+4, HCN, C2H2, HNC, and CO2. In every case a double zeta plus polarization (DZ+P) basis set of size C,N,O(9s5p1d/4s2p1d), H(4s1p/2s1p) was used. Previous analogous studies of H2O, H2CO, and NH3 are extended to form a statistical base of nine molecules. For these molecules 28 harmonic vibrational frequencies (out of total of 35 fundamentals) are thought to be known from experiment. The average errors with respect to experiment were found to be 9.1% (DZ+P self-consistent field), 3.7% (DZ+P configuration interaction including single and double excitations), and 2.2% (DZ+PCCSD). These statistics should provide guidance for the use of the CCSD method in situations where experimental vibrational frequencies are not available. Infrared intensities are also compared with available experimental data.