Accurate <i>ab initio</i> ro-vibronic spectroscopy of the X²&#8719; CCN radical using explicitly correlated methods

Explicitly correlated CCSD(T)-F12b calculations have been carried out with systematic sequences of correlation consistent basis sets to determine accurate near-equilibrium potential energy surfaces for the X&lt;sup&gt;2&lt;/sup&gt;&#8719; and a&lt;sup&gt;4&lt;/sup&gt;&#931;&lt;sup&gt;−&lt;/sup&gt; electronic states of the CCN radical. After including contributions due to core correlation, scalar relativity, and higher order electron correlation effects, the latter utilizing large-scale multireference configuration interaction calculations, the resulting surfaces were employed in variational calculations of the ro-vibronic spectra. These calculations also included the use of accurate spin-orbit and dipole moment matrix elements. The resulting ro-vibronic transition energies, including the Renner-Teller sub-bands involving the bending mode, agree with the available experimental data to within 3 cm&lt;sup&gt;−1&lt;/sup&gt; in all cases. Full sets of spectroscopic constants are reported using the usual second-order perturbation theory expressions. Integrated absorption intensities are given for a number of selected vibronic band origins. A computational procedure similar to that used in the determination of the potential energy functions was also utilized to predict the formation enthalpy of CCN, &#916;H&lt;sub&gt;f&lt;/sub&gt;(0K) = 161.7 &#177; 0.5 kcal/mol

On the electronic structure of small cyclic carbon clusters

We present the results of correlated calculations on a variety of small carbon rings. Equilibrium structures and vibrational frequencies are calculated and transition states connecting symmetry-equivalent minima are considered in detail. We show that neither single-reference coupled-cluster nor multiconfigurational self-consistent field methods (even after perturbational inclusion of dynamical correlation effects) give qualitatively correct potential surfaces in the vicinity of the minima, suggesting that there is little recourse for these systems other than a multireference coupled-cluster treatment. Density-functional theory using the B3LYP functional produces results broadly in agreement with single-reference coupled-cluster methods and is thus no more reliable, but considerably more economical. (c) 2008 Elsevier B.V. All rights reserved