A multiconfiguration self-consistent-field (MCSCF) study of the bent and linear conformations of HCCN

An MCSCF procedure of the complete active space (CAS) genre has been used to predict the structures and energetics of the HCCN molecule. Full configuration interaction (CI) within the π-electron space (1πx, 1πy, 2πx, 2πy, 3πx, 3πy orbitals, six electrons) was used in these MCSCF studies. With double zeta plus polarization (DZP) and triple zeta plus double polarization (TZ2P) basis sets, the bent structure lies 6.2 and 5.7 kcal, respectively, below the linear conformation. Multireference CI wave functions reduce this energy difference significantly.