In this paper and in a companion paper [M. E. Tuckerman and M. Parrinello, J. Chem. Phys. 101, 1316 (1994)] the problem of integrating the equations of motion in Car-Parrinello simulations is addressed. In this paper, new techniques for treating the constraint problem based on the velocity Verlet integrator and the Gaussian dynamics are presented. Questions of adiabaticity and temperature control are discussed, and it is shown how to combine the new techniques with the recently developed Nose-Hoover chain thermostat method. All new techniques are described using the formalism of operator factorizations applied to the classical Liouville propagator. In the companion paper, the formalism and application of multiple time scale methodology in Car-Parrinello simulations are discussed.