Ab Initio Calculations of Polarizabilities and Second Hyperpolarizabilities of Organic Molecules with Extended Pi-Electron Conjugation.

摘要

Static polarizability and second hyperpolarizability tensors are computed for a series of polyenes, polyynes, and cumulenes by ab initio SCF theory. Numerically stable finite field (FF) calculations can be achieved by using polynomial fits of either energy or induced dipole moment as a function of field strength. The nonlinear expansion coefficients from these fits correspond to the microscopic nonlinear optical property. Our results from fully coupled (FF) ab initio calculations for polarizability are in good agreement with those derived from uncoupled (sum-over-states) ab initio methods. The hyperpolarizabilities do not compare as well. A qualitative description of the chain length dependence of polarizability and hyperpolarizability for moderately long chains is discussed in terms of an empirical function. Diffuse orbital basis functions are required for qualitatively correct hyperpolarizabilities of small conjugated Pi systems or for that matter any small molecule. For example, the average second hyperpolarizability, gamma, of ethylene is computed to be -13, 1.7, and 726 au with STO-3G, 6-31G, and augmented 3-21G basis sets, respectively. We also describe the use of a corresponding orbital analysis to aid in the interpretation of ab initio results obtained by either FF or analytic derivative methods. Reprints. (aw)