Introduction

This chapter provides a concise introduction to the three foundational quantum frameworks in theoretical and computational chemistry: valence bond theory (VBT), molecular orbital theory (MOT), and density functional theory (DFT). Emphasis is placed on the chemical concepts developed within the two orbital-based approaches, VBT and MOT, as a prelude to the book's central focus, which is the systematic exposition of four density-based frameworks within DFT and their applications across a wide range of chemical phenomena and processes. These include bonding and nonbonding interactions, cooperativity and frustration, chirality hierarchy and homochirality, electrophilicity and nucleophilicity, steric effects and stereoselectivity, acidity and basicity, aromaticity and antiaromaticity, excited states, catalysis, and others. The book concludes with two integrative chapters: the first presents a general scheme for obtaining chemical understanding from the basic variables of theories, and the second extends this scheme to two emerging paradigms—machine learning and quantum computing.