Computational chemistry is a branch of chemistry that uses computer simulation to assist in solving chemical problems. It uses methods of theoretical chemistry, incorporated into computer programs, to calculate the structures and properties of molecules, groups of molecules, and solids. It is necessary because, apart from relatively recent results concerning the hydrogen molecular ion, the quantum many-body problem cannot be solved analytically, much less in closed form. While computational results normally complement the information obtained by chemical experiments, it can in some cases predict hitherto unobserved chemical phenomena. It is widely used in the design of new drugs and materials.
Quantum chemistry, also called molecular quantum mechanics, is a branch of chemistry focused on the application of quantum mechanics to chemical systems. Understanding electronic structure and molecular dynamics using the Schrödinger equations are central topics in quantum chemistry.
Theoretical chemistry is the branch of chemistry which develops theoretical generalizations that are part of the theoretical arsenal of modern chemistry: for example, the concepts of chemical bonding, chemical reaction, valence, the surface of potential energy, molecular orbitals, orbital interactions, and molecule activation.
The many-body problem is a general name for a vast category of physical problems pertaining to the properties of microscopic systems made of many interacting particles. Microscopic here implies that quantum mechanics has to be used to provide an accurate description of the system. Many can be anywhere from three to infinity, although three- and four-body systems can be treated by specific means and are thus sometimes separately classified as few-body systems. In such a quantum system, the repeated interactions between particles create quantum correlations, or entanglement. As a consequence, the wave function of the system is a complicated object holding a large amount of information, which usually makes exact or analytical calculations impractical or even impossible. Thus, many-body theoretical physics most often relies on a set of approximations specific to the problem at hand, and ranks among the most computationally intensive fields of science.
Coupled cluster (CC) is a numerical technique used for describing many-body systems. Its most common use is as one of several post-Hartree–Fock ab initio quantum chemistry methods in the field of computational chemistry, but it is also used in nuclear physics. Coupled cluster essentially takes the basic Hartree–Fock molecular orbital method and constructs multi-electron wavefunctions using the exponential cluster operator to account for electron correlation. Some of the most accurate calculations for small to medium-sized molecules use this method.
Electronic correlation is the interaction between electrons in the electronic structure of a quantum system. The correlation energy is a measure of how much the movement of one electron is influenced by the presence of all other electrons.
In computational chemistry, post–Hartree–Fock methods are the set of methods developed to improve on the Hartree–Fock (HF), or self-consistent field (SCF) method. They add electron correlation which is a more accurate way of including the repulsions between electrons than in the Hartree–Fock method where repulsions are only averaged.
Petr Čársky is a Slovakian quantum chemist.
Quantum Monte Carlo encompasses a large family of computational methods whose common aim is the study of complex quantum systems. One of the major goals of these approaches is to provide a reliable solution of the quantum many-body problem. The diverse flavors of quantum Monte Carlo approaches all share the common use of the Monte Carlo method to handle the multi-dimensional integrals that arise in the different formulations of the many-body problem.
Full configuration interaction is a linear variational approach which provides numerically exact solutions to the electronic time-independent, non-relativistic Schrödinger equation.
Rodney Joseph Bartlett is Graduate Research Professor of Chemistry and Physics, University of Florida, Gainesville, USA.
In quantum chemistry, size consistency and size extensivity are concepts relating to how the behaviour of quantum chemistry calculations changes with size. Size consistency is a property that guarantees the consistency of the energy behaviour when interaction between the involved molecular system is nullified. Size-extensivity, introduced by Bartlett, is a more mathematically formal characteristic which refers to the correct (linear) scaling of a method with the number of electrons.
Ab initio quantum chemistry methods are computational chemistry methods based on quantum chemistry. The term ab initio was first used in quantum chemistry by Robert Parr and coworkers, including David Craig in a semiempirical study on the excited states of benzene. The background is described by Parr. Ab initio means "from first principles" or "from the beginning", implying that the only inputs into an ab initio calculation are physical constants. Ab initio quantum chemistry methods attempt to solve the electronic Schrödinger equation given the positions of the nuclei and the number of electrons in order to yield useful information such as electron densities, energies and other properties of the system. The ability to run these calculations has enabled theoretical chemists to solve a range of problems and their importance is highlighted by the awarding of the Nobel prize to John Pople and Walter Kohn.
Anna I. Krylov is a Professor of Chemistry at the University of Southern California (USC), working in the field of theoretical and computational quantum chemistry. She is the inventor of the spin-flip method. Krylov is the president of Q-Chem, Inc. and an elected member of the International Academy of Quantum Molecular Science and the Academia Europaea.
Sourav Pal is an Indian theoretical chemist, a professor of chemistry in IIT Bombay, and is the director of the Indian Institute of Science Education and Research Kolkata. Pal was a director of CSIR-National Chemical Laboratory Pune, and adjunct professor at the Indian Institute of Science Education and Research Pune.
Piotr Piecuch is a Polish-born American physical chemist. He holds the title of University Distinguished Professor in the Department of Chemistry at Michigan State University, East Lansing, Michigan, United States. He supervises a group, whose research focuses on theoretical and computational chemistry as well as theoretical and computational physics, particularly on the development and applications of many-body methods for accurate quantum calculations for molecular systems and atomic nuclei, including methods based on coupled cluster theory, mathematical methods of chemistry and physics, and theory of intermolecular forces. His group is also responsible for the development of the coupled-cluster computer codes incorporated in the widely used GAMESS (US) package.
ORCA is an ab initio quantum chemistry program package that contains modern electronic structure methods including density functional theory, many-body perturbation, coupled cluster, multireference methods, and semi-empirical quantum chemistry methods. Its main field of application is larger molecules, transition metal complexes, and their spectroscopic properties. ORCA is developed in the research group of Frank Neese. The free version is available only for academic use at academic institutions.
Poul Jørgensen is professor of chemistry at the Department of Chemistry, Aarhus University (AU), Denmark and director of the qLEAP Center for Theoretical Chemistry at AU, which was established in April 2012. Jørgensen has made seminal contributions to the field of electronic structure theory. He is also one of the main authors of the DALTON program and a member of the International Academy of Quantum Molecular Science.
Péter R. Surján is a Hungarian theoretical chemist who is known for his research on application of the theory of second quantization in quantum chemistry. In 2016 a festschrift from Theoretical Chemistry Accounts journal was published in his name which is also published as a book in Highlights in Theoretical Chemistry series by the Springer Nature. He is currently the dean of the faculty of science in Eötvös Loránd University.
