Calculating dipole and quadrupole polarizabilities relevant to surface enhanced Raman spectroscopy

We have used ab initio electronic structure calculations to calculate the f requency dependent dipole-dipole polarizability (a), quadrupole-dipole pola rizability (A), quadrupole-quadrupole polarizability (C), (electric) dipole -magnetic dipole polarizability (G), and their normal coordinate derivative s for CO. The polarizability derivatives are of relevance to the interpreta tion of surface enhanced Raman (SER) spectra. Basis set convergence of thes e spectroscopic properties is studied, along with the effect of including e lectron correlation at the second-order polarization propagator (SOPPA) lev el, and the variation of the results with excitation frequency. The largest basis set SOPPA results that we have generated appear to be converged to w ithin 20% or better for most of the properties we have studied, however in a few cases the convergence is much poorer. The most difficult property to converge involves the off-diagonal component of the derivatives of the quad rupole-dipole polarizability tensor. Our results show that the ratio of the largest components of A to the largest components of a are on the order of one atomic unit in size, and a similar statement can be made concerning th e corresponding ratios of the normal coordinate derivatives. This means tha t the ratio of field derivatives to held strengths will also have to be on the order of one atomic unit in order for A and C to contribute comparably to cc in determining SERS intensities. (C) 1999 Elsevier Science B.V. All r ights reserved.