The Gaussian wave packet solution to the Schrödinger equation is studied for time-dependent Hamiltonians. The geometrical phase is obtained for a cyclic wave packet solution of the generalized harmonic oscillator with a nonadiabatic time-periodic Hamiltonian. It is found that the geometrical phase is independent ofh, and is equal to one-half of the classical nonadiabatic Hannay angle. The Hannay angle is shown to be independent of the classical action and does not involve averaging. [S0031-9007(97)02766-X] PACS numbers: 03.65.Bz, 03.65.GeSemiclassical Gaussian wave packets (GWP) have been applied to many problems in atomic and molecular physics including Franck-Condon spectra, photodissociation, spectral quantization, and Rydberg dynamics [1,2]. Most such applications apply to time-independent Hamiltonians, and the observation of cyclic wave packet has been of interest [2,3]. Wave-packet revival was also studied in the context of adiabatic time-dependent Hamiltonians, and the link between Berry's phase and Hannay's angle was confirmed [4]. The purpose of this Letter is to explore GWP dynamics for Hamiltonians with nonadiabatic time dependence, find conditions for the occurrence of cyclic GWP, and demonstrate an explicit connection between the nonadiabatic geometrical phase effects in quantum and classical mechanics. An ideal system for the study of timedependent GWP dynamics is the generalized harmonic oscillator, the adiabatic Hannay angle of which was studied in [5]. Since the time-independent counterpart of this system admits exact GWP solutions, it is natural to consider a time-dependent generalization. The relevance of this system is evident from its well-known special cases including Mathieu's and Hill's equations, which govern motions in the neighborhood of classical periodic orbits. The generalized harmonic oscillator Hamiltonian is H͑x, p x , t͒where ͑x, p x ͒ are the quantum operators corresponding to ͑q, p͒, the conjugate variables of the phase space, and the real parameters ͕a͑t͒, b͑t͒, c͑t͖͒ are periodic functions of time with a common period t. The exact GWP solution is extended to the time-dependent Hamiltonian (1), and a simple cyclic GWP is found. The original approach of Aharonov and Anandan [6] is generalized in order to facilitate our calculation of the geometrical phase. It is found that the geometrical phase of the cyclic GWP is equal to one-half of the nonadiabatic Hannay angle. It is also shown that the classical Hannay angle is independent of the action, and may be reduced to the area of a pair of ears, thus the ensemble averaging over action contour involved in the definition of Berry and Hannay [7] may be removed. These results indicate a unified formulation of the classical geometrical angle and quantum geometrical phase, which will be presented elsewhere [8].