Extended multireference electron correlation (MR-CISD, MR-CISD+Q, and MR-AQCC) calculations including full geometry optimization were performed on the valence-excited singlet (S1,S2) and triplet (T1-T4) states of acetylene. Large basis sets up to quadruple-zeta quality and basis set extrapolations were performed. Planar as well as nonplanar structures were calculated using a recently developed analytic gradient method for excited states. The harmonic vibrational frequencies were also computed. Several interesting intersections between different electronic states (S1/S2, T2/T3, and S1/T3) were characterized.