Direct atomic-orbital-based time-dependent Hartree-Fock calculations of frequency-dependent polarizabilities

We have formulated and implemented a direct atomic integral driven method for the calculation of frequency-dependent response properties at the self-consistent-field level. By avoiding the integral transformation step, as well as the storing and retrieving of atomic-orbital-based integrals, we are able to use large basis sets. The practicality of the approach is illustrated and calibrated by performing a series of calculations on cyclopropenone employing up to 232 basis orbitale. We examined the scaling of the dipole polarizability (α) with the size of the system for paranitroaniline and its dimer. Except for a small positive enhancement of the component along the molecular axis, we find little effect of size on α for this system. However, if the -NN- linkage of the dimer is replaced by a -CC- linkage, thus more effectively extending the π-orbital conjugation by making the dimer planar, we find a large, frequency-dependent increase in the polarizability relative to twice that of the monomer (factors varying from 3 to 18, depending on frequency). This makes the -CC- linked polymer a potential candidate for achieving nonlinear chain length dependence of properties that depend on α.