Vibrational Stark spectroscopy 3. Accurate benchmark ab initio and densityfunctional calculations for CO and CN-

A new Stark-type experimental technique known as electroabsorption spectros copy has been developed in S. G. Boxer's laboratories which offers the abil ity to measure the responses of isotropic solvated molecules to an applied electric field. It is applicable to a wide range of molecules and environme nts. We present benchmark ab initio and density-functional calculations emp loying 14 methods and double-zeta to penta-zeta augmented basis sets for th e vibrational frequencies, dipole moment, polarizability, hyperpolarizabili ty profiles, Stark shift, and other electroabsorption properties of two imp ortant (gas-phase) molecules, CO and CN-, for which a range of high-precisi on experimental data is available. The results show excellent agreement wit h these data, verify the earlier conclusion that the transition-moment pola rizability rather than the polarizability change dominates the first-deriva tive response in the electroabsorption spectroscopy of these systems, and c onvergence with respect to the treatment of electron correlation, basis set , geometrical integration, and finite-field differentiation. For large mole cules, B3LYP calculations with small (eg., aug-cc-pVDZ) basis sets are pred icted to offer optimum performance per computational cost.