The LNCS journal Transactions on Computational Science reflects recent developments in the field of Computational Science, conceiving the field not as a mere ancillary science but rather as an innovative approach supporting many other scientific disciplines. The journal focuses on original high-quality research in the realm of computational science in parallel and distributed environments, encompassing the facilitating theoretical foundations and the applications of large-scale computations and massive data processing. It addresses researchers and practitioners in areas ranging from aerospace to biochemistry, from electronics to geosciences, from mathematics to software architecture, presenting verifiable computational methods, findings and solutions and enabling industrial users to apply techniques of leading-edge, large-scale, high performance computational methods. The 7th issue of the Transactions on Computational Science journal is devoted to core computational science techniques, such as grid computing, advanced numerical methods, and stochastic systems. It has been divided into two parts. The five papers in Part I focus on computations of stochastic systems and the four papers in Part II focus on computational methods for complex systems.