The potential energy surface and dipole moment surfaces of the ã 4 A 2 electronic state of CH + 2 are calculated ab initio using an augmented correlation-consistent polarized valence quadruple-ζ (aug-cc-pVQZ) basis set, with the incorporation of dynamical correlation using the coupled cluster method with single and double excitations and perturbatively connected triple excitations [CCSD(T)]. We use these surfaces in the MORBID program system to calculate rotation and rotation-vibration term values for ã-state CH + 2 , CD + 2 , and CHD + and to simulate the rotation and rotation-vibration absorption spectrum of CH + 2 in the ã 4 A 2 electronic state. Our work is motivated by studies of CH + 2 that use the Coulomb explosion imaging technique and by the goal of predicting spectra that may be obtained from discharge sources. Although the ã state is the lowest-lying excited state above the X / Ã ground state pair, it turns out to be relatively high-lying, and we determine that T e ( ã) = 30447.5 cm -1 . The equilibrium bond angle for ã-state CH + 2 is only 77.1 • ; as a result the asymmetric top κ value is close to 0, and the molecule is equally far from the oblate and prolate symmetric top limits in this electronic state.