ICN and ICN− have been studied at the MR-SO-CISD level of theory with triple-zeta basis sets for all three atoms. The potential surfaces for the ground states of ICN and ICN− as well as for the first five excited states of ICN− have been generated from the electronic energies, and properties of these states are described. The minimum energy geometry of ICN− is linear, with a local minimum in the INC− geometry. The zero-point corrected energy difference between these two isomers is 0.38 eV, and they are separated by a 0.5 eV barrier. The I···CN(COM) equilibrium distances are 3.27 and 3.14 Å in the ICN− and INC− geometries, respectively. These values are 0.6 Å larger than the I···CN distances in the corresponding minima in the ICN potential. Likewise, the zero-point amplitude of both the I···CN stretch and bend are much larger in ICN− than in ICN. This is captured by the calculated anharmonic fundamental frequencies for ICN− of 70 and 235 cm−1 for the bend and stretch, compared to anharmonic frequencies of 302 and 488 cm−1 for the bend and stretch fundamentals in ICN. The frequencies are lowered further in INC− where the bend and stretch fundamentals have frequencies of 59 and 220 cm−1. © 2012 Wiley Periodicals, Inc.