Density functional theory (DFT) has been used to calculate the vibrational Stark tuning rates of a variety of nitriles and carbonyls in quantitative agreement with experimental values with a correction factor of f=1.1 for the local electric field. These calculations show that the vibrational Stark tuning rate has an anharmonic contribution and a contribution due to geometric distortions caused in the molecules due to the applied electric field. The anharmonic and geometric distortion components of the vibrational Stark tuning rate were calculated by the frequency dependence of the CN or CO stretching mode with varying applied electric fields by using the optimized structure in zero applied field or allowing the structure to optimize in the applied electric field, respectively. The changes in the calculated frequency of the CN or CO stretching mode, bond length, and dipole moment of this bond with varying applied electric fields are shown. The transition polarizability and the difference polarizability were also calculated by DFT for comparison to the experimental data on nitriles and carbonyls. The DFT calculations suggest that the sign of the transition polarizability is negative and this result in turn has an effect on the experimental data analysis since the sign of the transition polarizability is not determined by experiment. (C) 2003 American Institute of Physics.