Modeling Nonlinear Optical Properties of Transition Metal Complexes. Basis Set, Effective Core Potential, and Geometry Effects

Nonlinear optical (NLO) properties of transition metal complexes are studied using quantum chemical calculations. By comparison with all electron calculations, effective core potentials have been shown to be competent for the calculation of NLO properties as long as the valence basis sets are comparable. While overall the basis set effects are important for calculation of NLO properties, they are found to be less important for the central transition metal than for the surrounding ligands. Augmenting the basis set of main group elements with diffuse s, p, and d functions in a proper way could provide the best compromise between speed and accuracy of the computation. Interesting trends are found in the calculation of NLO properties of [MO4]. Both polarizability (R) and second hyperpolarizability (γ) decrease toward the right across the transition series. The second series [MO4] have the largest R among the three metalates in a triad. For group IVB and VB complexes with larger charges (-4 and -3, respectively), the second series [MO4] have the largest γ, while for groups VIB, VIIB, and VIII, with less anionic metalates (-2, -1, and 0, respectively), the third series metalates have the largest γ. The relative difference in both R and γ values among the three series in the same group is much smaller than between different groups. Overall, variations in the calculated values of NLO properties correlate with M-O bond lengths and hence the size of the metalate ion.