"In this proposal, we develop a cutting-edge computational tool for simulating molecular dynamics at the quantum level that will be applied to understand the dynamics of various chemical phenomena, especially for hydrogen-transfer reactions.Hydrogen-transfer reactions are one of the most basic elementary chemical reactions, and play vital roles in a number of industrially or biologically intriguing systems. Despite their importance, experimental studies of hydrogen-transfer reactions have faced several difficulties. Experimentally, it is difficult to observe hydrogen-transfer reactions directly, owing to their short time scale. In addition, the analysis of experimental results is often challenging, because the current interesting systems are usually large systems (e.g., enzymes).There is thus a strong demand for computational software that can accurately simulate hydrogen-transfer reactions. At this moment, all existing molecular dynamics methods do not satisfy this demand. The methods that can precisely describe hydrogen-transfer reactions are only applicable for small systems. Others suffer from unreliability owing to neglected or approximated quantum effects.In this context, we propose a novel ab initio vibrational wave-function theory to describe general chemical reactions in large molecules with a conclusive accuracy. We call this the Vibrational Active Space Second-order Perturbation Theory (VASPT2). This method exploits a variational method for strong quantum effects among small degrees of freedom, and employs a perturbative method for weak quantum effects in a whole system to achieve quantitative results. The proposed method compromises the applicability and reliability (accuracy) by a well-balanced manner. As an application of this method, we will describe the hydrogen-transfer reaction of AADH (aromatic amine dehydrogenase) to answer the question how quantum tunneling effects are important in enzymes."