A novel approach for modeling of quantum dynamics and spectroscopy in 
large polyatomic systems is presented. The recently developed Classical
Separable Potential (CSP) method propagates a multidimensional separable quantum
wavepacket using effective time-dependent single 
mode potentials guided by
classical trajectories. In this way, couplings between individual 
degrees of freedom
are taken into account in a mean-field approximation. 
Comparison with 'numerically exact' calculations for
small systems and with experiments on large systems show that the CSP
scheme is accurate and suitable for the description of short timescale
processes in the context of dynamical and 
spectroscopic applications. Moreover, calculations for systems
having up to 1000 coupled modes on scalar and up to 10 000 degrees
of freedom on parallel computers are not computationally
extremely demanding. This paper presents 
the program package QDYN which implements the CSP method.