Knowledge Bank

Ab initio SCF and CI calculations have been performed for the ground and lowest excited states of the $C2H$ radical. A medium-sized Gaussian basis set was used (double zeta and polarization functions and diffuse s-functions) and an iterative natural orbital technique adopted for the determination of weakly occupied valence shell orbita1s. $C2H$ has a low-lying $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ state, about 0.5 eV $(T00)$ above the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Sigma +$ ground state, whereas all $\pi \pi \ast$ excited states are much higher $(T00>6 eV)$. In addition to vertical and adiabatic excitation energies, equilibrium geometries and vibrationa1 frequencies have been determined for a number of excited states.