Valence configuration interaction wavefunctions are used to calculate dipole moment derivatives and force constants for linear hydrogen cyanide. The magnitude of the dipole moment is found to decrease when the C-N bond is stretched from its equilibrium value. The implications for hydrogen bonding in crystalline HCN are discussed. Self-consistent field calculations are used to confirm that the SCF dipole derivative with respect to the CN bond length has the wrong sign and to explore the sensitivity of the dipole derivatives to the choice of atomic basis set. © 1978 American Institute of Physics.