Quantum chemistry
Quantum chemistry
Quantum chemistry
The underlying physical laws that are necessary for the mathematical theory of a large part of physics and all of chemistry are therefore completely known, and the only difficulty is that the exact application of these laws leads to equations that are far too complicated to be solvable (P. A. M. Dirac)
Even before Dirac got to the heart of the problem in 1929 , it was clear to researchers in the subject of chemistry that classical physics could not describe the behaviour of matter on the smallest scale - at the level of atoms and electrons. New, sometimes bizarre concepts emerged: Energy quanta, wave-particle duality, uncertainty principle. With the formulation of the Schrödinger equation in the mid-1920s, the fundamental mathematical description for the microscopic world seemed to have been found.
The question was formulated.
Quantum mechanics: A promising theory for chemistry
Physicists were soon able to successfully describe simple systems such as the hydrogen atom and although Heitler wrote as early as 1931:"The latest fruit of physical research, quantum mechanics, is now celebrating its fifth anniversary. It claims to control the entire phenomena of the atomic world, and the chemist [hic!] must familiarise himself with the results of quantum mechanics if he wants to pursue his science as fruitfully as possible.", quantum mechanics remained a promising but inapplicable theory for chemistry for a long time.
Chemical systems, even small molecules, consist of many electrons and atomic nuclei that interact with each other. The exact(!) solution of the quantum mechanical equations for such multi-particle systems was - and in principle still is today for large systems - computationally impossible. The answer to Dirac's question seemed a long way off.
The fact that quantum mechanics ultimately found application in chemistry is thanks to pioneers of computer development such as Conrad Zuse, who developed the world's first functional digital computer, the Zuse Z3. Over the decades, computers achieved computing capacities that allowed scientists to at least approximately solve the quantum mechanical equations for complex chemical systems. This was the birth of modern quantum chemistry.
Quantum chemistry: An application of quantum mechanics to chemical problems
Chemistry is often understood as the science behind the questions,
- how atoms react with each other and combine to form molecules,
- how molecules interact with each other and
- what properties the resulting matter possesses.
To this day, we often rely on models and concepts whose inherent problem 1976 G. Box so aptly formulated in:"In principle, all models are wrong, but some are useful.".
Valence bond theory, Lewis formulae and mesomeric boundary structures attempt to describe the electron distribution in a molecule using "pictures". In quantum chemical terms, these are often just single terms or contributions to a much more complex overall wave function that encompasses all possible electron states simultaneously.
Computational quantum chemistry now provides us with the tools to look deeper. It calculates the wave function and the properties derived from it, such as energies, geometries, charge distributions or spectroscopic fingerprints, directly from the fundamental laws of quantum mechanics. The Hartree-Fock method transforms the problem of solving the Schrödinger equation for multi-electron systems into an algebraic problem in matrix form that is easy for computers to handle and can be solved iteratively. This method forms the quantitative theoretical foundation of the orbital model often used throughout chemistry.
However, the Hartree-Fock method (and thus the orbital model) is not sufficient for the exact description of chemical problems. Although this approach already captures 99% of the exact energetics for many systems, methods that go beyond the orbital model must be developed and applied for the description of systems with chemical accuracy. These so-called post-Hartree-Fock methods (either wave-function based or in the framework of density functional theory) allowus today not only to interpret experimental results, but also to predict and understand chemical systems and processes that are difficult to access experimentally.
The simulation of a Diels-Alder reaction with the Gaussian programme package
Quantum Chemistry at the Carl von Ossietzky Universität Oldenburg
The research focus of quantum chemistry at the Institute of Chemistry at the Carl von Ossietzky Universität Oldenburg is on predicting and understanding chemical systems and processes, especially those that are difficult to access. This includes the simulation of dynamic processes such as laser-induced desorption and the reaction of molecules on solid surfaces.
Through the development of a high-dimensional wave packet code we can precisely model the motion of atoms and molecules on potential hypersurfaces. This allows the investigation of complex non-adiabatic effects and the influence of external fields such as laser pulses and the description of open quantum systems (taking into account energy dissipation and decoherence) and the optimal control of these systems.
The simulations, based on quantum mechanical ab initio calculations of the potential energy surfaces enable a fundamental mechanistic understanding of experimental observations. The further development and parallelisation of these quantum mechanical simulation methods on high-performance computers allows the investigation of increasingly complex systems and opens up new insights into fundamental chemical processes on solid surfaces.
