Density-functional theory studies of the interfacial boundaries in soft and hard condensed matters and development of density-functional many-body van der waals correction method = 밀도범함수 이론 기반 응집물질의 계면 연구 및 다체 반데르발스 상호작용 보정법의 개발
Interfacial boundaries in soft and hard condensed matters play important role in governing macroscopic properties of materials due to their stability properties depending on their structures. Therefore, it is crucial to understand the physical and mechanical properties of interfacial boundaries embedded in bulk, and the interactions between different types of boundaries or between the boundaries and other defects commonly observed in condensed phase. In this thesis, we employ first-principles density functional theory to examine various physical and chemical phenomena observed at the interfacial boundaries. They are paraelectric-ferroelectric phase transition in the PVDF polymers, grain boundary segregation of alloying elements in steels, twinning deformations of magnesium alloys, and catalytic activity of gold surface containing grain boundaries for the electrochemical $CO_2$ reduction. In such studies, the correction of van der Waals interactions, of which description is often failed in density functional theory within the (semi-)local approximation of the exchange-correlation functional, is important in understanding the interatomic or intermolecular interactions. Therefore, we tested the performance of currently available density functional van der Waals correction methods for various systems covering molecular gases, molecular crystals, ionic crystals, layered materials, and bulk metallic crystals. From the tested results, we found that these methods are difficult to describe van der Waals interactions in metallic systems, and we suggested a new many-body density functional van der Waals correction method based on the smeared-dipole model to solve this problem.