Complete active space self-consistent field (CASSCF) and complete active space second-order perturbation theory (CASPT2) calculations in conjunction with the aug-cc-pVTZ basis set have been used to investigate the low-lying electronic states of thiofulminic acid (HCNS), HCNS+, and HCNS−. The result of geometry optimization using CASPT2/aug-cc-pVTZ shows that theoretically determined geometric parameters and harmonic vibrational frequencies for the HCNS ground state X1Σ+(X1A′) are in agreement with previous studies. The ionization energies, the electron affinity energies, the adiabatic excitation energies, and vertical excitation energies have been calculated and the corresponding cation and anion states are identified. By calculating adiabatic electron affinity, the states of HCNS− have been identified to contain both π orbital states (X2A′ and 12A″) and dipole-bond states (14A′ and 14A″). © 2012 Wiley Periodicals, Inc.