Development, implementation and applications of hybrid quantum chemistry models

Abstract

The field of theoretical quantum chemistry aims to organize chemical laws, principles and rules by applying quantum mechanics to various challenges within the real of chemistry. Central to this framework is the interconnection of the structure with the properties of molecular systems. This interconnection is used to provide theories and explanations for chemical phenomena, such as bonding, heats of formation, and reactivity mechanisms, among others. From the early days of quantum mechanics, electronic structure theory establishes the fundamental laws necessary for the mathematical treatment of chemistry problems. These laws are well-established and understood. However, the enormous task of method development is oriented towards overcoming the limitations imposed by the large computational cost involved in the solutions to the many-body electronic Schrödinger equation. Achieving accurate or exact solutions for this equation allows for a complete description of the motions of electrons in atoms and molecules, posing a formidable challenge. Although theoretically feasible, in practice, the exact solution is prohibitively expensive to compute for large systems. Thus, designing a method that delivers cheap and accurate approximate solutions is primordial for the development of theoretical chemistry and to extend accurate solutions to large systems. In developing such methods, the description of electronic correlation is of fundamental importance. This refers to the influence that the motion of one electron has on the motion of another electron in a many-electron system. Central to this description is the concept of electron correlation energy, defined as a measure for the energy difference between the exact solution and that captured by mean field models (e.g., Hartree-Fock).