For molecules in high intensity oscillating electric fields, the time-dependent Hartree-Fock (TDHF) method is used to simulate the behavior of the electronic density prior to ionization. Since a perturbative approach is no longer valid at these intensities, the full TDHF equations are used to propagate the electronic density. A unitary transform approach is combined with the modified midpoint method to provide a stable and efficient algorithm to integrate these equations. The behavior of H2+ in an intense oscillating field computed using the TDHF method with a STO-3G basis set reproduces the analytic solution for the two-state coherent excitation model. For H2 with a 6-311++G(d,p) basis set, the TDHF results are nearly indistinguishable from calculations using the full time-dependent Schrödinger equation. In an oscillating field of 3.17 x 10(13) W cm(-2) and 456 nm, the molecular orbital energies, electron populations, and atomic charges of H2 follow the field adiabatically. As the field intensity is increased, the response becomes more complicated as a result of contributions from excited states. Simulations of N2 show even greater complexity, yet the average charge still follows the field adiabatically.

Recent advances in nonlinear optics and strong-field chemistry highlight the need for calculated properties of organic molecules and their molecular ions for which no experimental values exist. Both static and frequencydependent properties are required to understand the optical response of molecules and their ions interacting with laser fields. It is particularly important to understand the dynamics of the optical response of multielectron systems in the near-IR (λ ∼ 800 nm) region, where the majority of strong-field experiments are performed. To this end we used Hartree-Fock (HF) and PBE0 density functional theory to calculate ground-state firstorder polarizabilities (R) for two series of conjugated organic molecules and their molecular ions: (a) alltrans linear polyenes ranging in size from ethylene (C 2 H 4 ) to octadecanonene (C 18 H 20 ) and (b) polyacenes ranging in size from benzene (C 6 H 6 ) to tetracene (C 18 H 12 ). The major observed trends are: (i) the wellknown nonlinear increase of R with molecular size, (ii) a significant increase of R upon ionization for larger systems, and (iii) for larger ions, the dynamic polarizability at 800 nm is much larger than the static polarizability. We have also compared the HF and PBE0 polarizabilities of the linear polyenes up to octatetraene calculated with second-order Moller-Plesset perturbation theory (MP2) and coupled cluster theory with single and double excitations (CCSD). For neutral molecules the results at the PBE0 and HF levels are very similar and ca. 20% higher than the MP2 and CCSD results. For molecular ions, results at the HF, PBE0, MP2, and CCSD are all very close. We discuss the size scaling and frequency dependence of R, and provide simple models that capture the origin of the change in the static and dynamic polarization upon ionization.