Ab Initio calculations and vibrational energy level fits for the lower singlet potential-energy surfaces of C3.

Ab initio multireference configuration interaction potential energy surfaces are computed for the eight lowest singlet surfaces of C₃. These reveal several important features, including several conical intersections in linear, nonlinear, and equilateral triangle geometries. These intersections are important because, particularly for the excited à ¹Π_u state, reasonable ab initio results could only be obtained by including nearby, near degenerate, ¹Σ_u^- and ¹Δ_u states that cross the à ¹Π_u state around 4500 cm-1 above the equilibrium geometry, and a ¹Π_g state whose potential in turn crosses the other states about 2000 cm-1 further up. These states are probably responsible for the complexity of the shorter wavelength UV absorption spectrum of C₃. The computed potential energy surface for the ground, X ¹Σ_g⁺, state and for the lowest two excited singlet surfaces (which both correlate with the à ¹Π_u state in a collinear geometry) are fitted to analytic functional forms. Vibrational energy levels are calculated for both states, taking account of the Renner-Teller coupling in the excited à ¹Π_u state. The potential parameters for both states are then least-squares fitted to experimental data. The ground-state fit covers a range of ∼8500 cm-1 above the lowest level, and reproduces 100 observed vibrational levels with an average error of 2.8 cm-1. The à ¹Π_u state surfaces cover a range of 3250 cm-1 above the zero-point level, and reproduce the 44 observed levels in this range with an average error of 2.8 cm-1. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]