Force constants for HCN have been determined from both the one-electron model, SCF, and correlated, CI, wavefunctions. Using SCF wavefunctions, the stretching force constant for the CH bond, K11, is 9% larger than experimentally determined values and the CN stretch, K22 is 23% larger. When electron correlation is included through the use of configuration interaction wavefunctions the agreement of computed and experimental force constants is greatly improved; K11 is 5% larger than the experimental value and K22 is 9% larger. The force constants are determined by making least square polynomial fits to computed points on the potential surface. Force constants for both stretch and bending modes are reported through quartic terms. Copyright © 1975 American Institute of Physics.