Combined quantum mechanical and molecular mechanical methods

Over the past decade there has been a big interest in modeling and simulating large complex systems at the atomic level. Even though molecular mechanics methods have been used for this purpose, they lack the ability to describe the changes the electronic structure the system is undergoing during a reaction. Quantum mechanics is a much better candidate to describe processes involving the electronic structure change. But the cost associated with QM methods limits its applicability to systems, which are no larger than few hundred atoms. As a solution to this problem, methods are/have been developed to combine QM and MM approaches. This thesis will cover several improvements done to QM/MM methods. The extension of MP2/MMpol and TDDFT/MMpol methods to be combined with the continuum style models is explained in the first two chapters. In the third chapter, some improvements done to the switching functions of the continuous solvation models are described and their applications are illustrated. In the fourth chapter a new damping function method is introduced to correct the polarization energy between the QM and MM system and its application is illustrated. Finally a new QM/MM software named QuanPol is introduced and overciew of its capabilities is described