From Whelped, the free encyclopedia Computational chemistry Is a branch of chemistry that uses principles of computer science to assist In solving chemical problems. It uses the results of theoretical chemistry, incorporated into efficient computer programs, to calculate the structures and properties of molecules and solids.
Its necessity arises from the well-known fact that apart from relatively recent results concerning the hydrogen molecular ion (see references therein for more details), the quantum many-body problem cannot be loved analytically, much less in closed form. While its 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. Examples of such properties are structure (I. E. He expected positions of the constituent atoms), absolute and relative (Interaction) energies, electronic charge distributions, dipoles higher multiple moments, vibration frequencies, reactively or other spectroscopic quantities, and cross sections for collision with other particles. The methods employed cover both static and dynamic situations. In all cases the computer time and other resources (such as memory and disk space) increase rapidly with the size of the system being studied. That system can be a single molecule, a group of molecules, or a solid.
Computational chemistry methods range from highly accurate to very approximate; highly accurate methods are typically feasible only for small systems. ABA monition methods are based entirely on theory from first principles. Other (typically less accurate) methods are called empirical or semi- empirical because they employ experimental results, often from acceptable models of atoms or related molecules, to approximate some elements of the underlying theory. Both ABA Inlets and semi-empirical approaches Involve approximations.
These range from simplified forms of the first-principles equations that are easier or faster to solve, to approximations limiting the size of the system (for example, periodic boundary conditions to fundamental approximations to the underlying equations that are required to achieve any solution to them at all. For example, most ABA monition calculations make the Born-Oppenheim approximation, which greatly amplifies the underlying ScarГ¶dinner equation by assuming that the nuclei remain in place during the calculation.
In principle, ABA monition methods eventually converge to the exact solution of the underlying equations as the number of approximations is reduced. In practice, however, it is impossible to eliminate all approximations, and residual error Inevitably remains. The goal of computational chemistry is to minimize this residual error while keeping the calculations tractable. In some cases, the details of electronic structure are less important than the long-time phase space behavior of lossless. This Is the case In conformational studies of proteins and protein-lagans binding thermodynamics.
Classical approximations to the potential energy surface to enable longer simulations of molecular dynamics. Furthermore, semiautomatics uses even more empirical (and computationally cheaper) methods like machine learning based on physiochemical properties. One typical problem in semiautomatics is to predict the binding affinity of drug molecules too given target. Contents [hide] * 1 History * 2 Fields of application * 3 Accuracy * 4 Methods * 4. 1 ABA monition methods * 4. 2 Density functional ethos * 4. Semi-empirical and empirical methods * 4. 4 Molecular mechanics * 4. 5 Methods for solids * 4. 6 Chemical dynamics * 4. 7 Molecular dynamics * 5 Interpreting molecular wave functions * 6 Software packages * 7 See also * 8 Cited references * 9 Other references * 10 Specialized Journals on computational chemistry * 11 External links I History Building on the founding discoveries and theories in the history of quantum mechanics, the first theoretical calculations in chemistry were those of Walter Hitler and Frizz London in 1927.
The books that were influential in the early placement of computational quantum chemistry include Lines Palling and E. Bright Willow’s 1935 Introduction to Quantum Mechanics – with Applications to Chemistry, Erring, Walter and Kimball 1944 Quantum Chemistry, Whittler’s 1945 Elementary Wave Mechanics – with Applications to Quantum Chemistry, and later Scullion’s 1952 textbook Valence, each of which served as primary references for chemists in the decades to follow.
With the development of efficient computer technology in the ass’s, the solutions of elaborate wave equations for complex atomic systems began to be a realizable objective. In the early ass’s, the first semi-empirical atomic orbital calculations were carried out. Theoretical chemists became extensive users of the early digital computers. A very detailed account of such use in the United Kingdom is given by Smith and Stultifies.  The first ABA monition Heartier-Bock calculations on diatomic molecules were carried out in 1956 at MIT, using a basis set of Slater orbital.
For diatomic molecules, a systematic study using a minimum basis set and the first calculation with a larger basis set were published by Arians and Nesses respectively in 1960. 2] The first polyatomic calculations using Gaussian orbital were carried out in the late ass’s. The first configuration interaction calculations were carried out in Cambridge on the DEEDS computer in the ass’s using Gaussian orbital by Boys and coworkers.  By 1971, when a bibliography of ABA monition calculations was published, the largest molecules included were naphthalene and ukulele. 5] Abstracts of many earlier developments in ABA monition theory have been published by Schaefer.  In 1964, HГјcell method calculations (using a simple linear combination of atomic orbital (ALCOA) method for the determination of electron energies of molecular orbital of electrons in conjugated hydrocarbon systems) of molecules ranging in complexity from butadiene and benzene to abalone, were generated on computers at Berkeley and Oxford. These empirical methods were replaced in the ass’s by semi- empirical methods such as CONDO. 9] In the early ass’s, efficient ABA monition computer programs such as TOMATO, Gaussian, BIMBO, and POLYMATH, began to be used to GAUSSIAN, now massively expanded, is still in use, but many other programs are now in use. At the same time, the methods of molecular mechanics, such as MM, were developed, primarily by Norman Linger.  One of the first mentions of the term “computational chemistry” can be found in the 1970 book Computers and Their Role in the Physical Sciences by Sidney Branch and Abraham Haskell Daub, where they state “It seems, therefore, that ‘computational chemistry’ can finally be more and more of a reality. [1 1] During the ass’s, widely different methods began to be seen as part of a new emerging discipline of computational chemistry.  The Journal of Computational Chemistry was first published in 1980. Fields of application The term theoretical chemistry may be defined as a mathematical description of chemistry, whereas computational chemistry is usually used when a mathematical method is sufficiently well developed that it can be automated for implementation on a computer.
In theoretical chemistry, chemists, physicists and mathematicians develop algorithms and computer programs to predict atomic and molecular properties and reaction paths for chemical reactions. Computational chemists, in contrast, may simply apply existing computer programs and methodologies to specific chemical questions. There are two different aspects to computational chemistry: * Computational studies can be carried out to find a starting point for a laboratory synthesis, or to assist in understanding experimental data, such as the position and source of spectroscopic peaks. Computational studies can be used to predict the possibility of so far entirely unknown molecules or to explore reaction mechanisms that are not readily studied by experimental means. Thus, computational chemistry can assist the experimental chemist or it can challenge the experimental chemist to find entirely new chemical objects. Several major areas may be distinguished within computational chemistry: * The prediction of the molecular structure of molecules by the use of the simulation of forces, or more accurate quantum chemical methods, to find stationary points on the energy surface as the position of the nuclei is varied. Storing and searching for data on chemical entities (see chemical databases). * Identifying correlations between chemical structures and properties (see ESP. and QUASAR). * Computational approaches to help in the efficient synthesis of compounds. * Computational approaches to design molecules that interact in pacific ways with other molecules (e. G. Drug design and catalysis). Accuracy The words exact and perfect do not appear here, as very few aspects of chemistry can be computed exactly. However, almost every aspect of chemistry can be described in a qualitative or approximate quantitative computational scheme.
Molecules consist of nuclei and electrons, so the methods of quantum mechanics apply. Computational chemists often attempt to solve the non- relativistic ScarГ¶dinner equation, with relativistic corrections added, although some progress has been made in solving the fully relativistic Doric equation. In principle, it independent form, as appropriate for the problem in hand; in practice, this is not possible except for very small systems. Therefore, a great number of approximate methods strive to achieve the best trade-off between accuracy and computational cost.
Accuracy can always be improved with greater computational cost. Significant errors can present themselves in ABA monition models comprising many electrons, due to the computational expense of full relativistic-inclusive methods. This complicates the study of molecules interacting with high atomic mass unit atoms, such as transitional metals and their catalytic properties. Present algorithms in computational chemistry can routinely calculate the properties of molecules that contain up to about 40 electrons with sufficient accuracy.
Errors for energies can be less than a few k/mol. For geometries, bond lengths can be predicted within a few viscometers and bond angles within 0. 5 degrees. The treatment of larger molecules that contain a few dozen electrons is computationally tractable by approximate methods such as density functional theory (DAFT). There is some dispute within the field whether or not the latter methods are sufficient to describe complex chemical reactions, such as hose in biochemistry. Large molecules can be studied by semi-empirical approximate methods.
Even larger molecules are treated by classical mechanics methods that employ what are called molecular mechanics. In CM/MM methods, small portions of large complexes are treated quantum mechanically (CM), and the remainder is treated approximately (MM). Methods A single molecular formula can represent a number of molecular isomers. Each isomer is a local minimum on the energy surface (called the potential energy surface) created from the total energy (I. E. , the electronic energy, plus the repulsion energy twine the nuclei) as a function of the coordinates of all the nuclei.
A stationary point is a geometry such that the derivative of the energy with respect to all displacements of the nuclei is zero. A local (energy) minimum is a stationary point where all such displacements lead to an increase in energy. The local minimum that is lowest is called the global minimum and corresponds to the most stable isomer. If there is one particular coordinate change that leads to a decrease in the total energy in both directions, the stationary point is a transition structure and the coordinate is the reaction coordinate.
This process of determining stationary points is called geometry optimization. The determination of molecular structure by geometry optimization became routine only after efficient methods for calculating the first derivatives of the energy with respect to all atomic coordinates became available. Evaluation of the related second derivatives allows the prediction of vibration frequencies if harmonic motion is estimated. More importantly, it allows for the characterization of stationary points. The frequencies are related to the sunglasses of the Hessian matrix, which contains second derivatives.
If the sunglasses are all positive, then the frequencies are all real and the stationary point is a local minimum. If one generally is negative (I. E. , an imaginary frequency), then the stationary point is a transition structure. If more than one generally is negative, then the stationary point is a more complex one, and is usually of little interest. When experimenter is looking solely for local minima and transition structures. The total energy is determined by approximate solutions of the time-dependent ScarГ¶dinner equation, usually with no relativistic terms included, and by making use of the Born-
Oppenheim approximation, which allows for the separation of electronic and nuclear motions, thereby simplifying the ScarГ¶dinner equation. This leads to the evaluation of the total energy as a sum of the electronic energy at fixed nuclei positions and the repulsion energy of the nuclei. A notable exception are certain approaches called direct quantum chemistry, which treat electrons and nuclei on a common footing. Density functional methods and semi-empirical methods are variants on the major theme. For very large systems, the relative total energies can be compared using molecular mechanics.
The ways of determining the total energy to predict molecular structures are: ABA monition methods Main article: ABA monition quantum chemistry methods The programs used in computational chemistry are based on many different quantum-chemical methods that solve the molecular ScarГ¶dinner equation associated with thunderclap Hamiltonian. Methods that do not include any empirical or semi-empirical parameters in their equations – being derived directly from theoretical principles, with no inclusion of experimental data – are called ABA monition methods.
This does not imply that the solution is an exact one; they are all approximate quantum mechanical calculations. It means that a particular approximation is rigorously defined on first principles (quantum theory) and then solved within an error margin that is qualitatively known beforehand. If numerical iterative methods have to be employed, the aim is to iterate until full machine accuracy is obtained (the best that is possible with a finite word length on the computer, and within the mathematical and/or physical approximations made).
Diagram illustrating various ABA monition electronic structure methods in terms of energy. Spacing are not to scale. The simplest type of ABA monition electronic structure calculation is the Heartier-Bock (HP) scheme, an extension of molecular orbital theory, in which the correlated electron-electron repulsion is not specifically taken into account; only its average effect is included in the calculation. As the basis set size is increased, the energy and wave function tend towards a limit called the Heartier-Bock limit.
Many types of calculations (known as post-Heartier-Bock methods) begin with a Heartier-Bock calculation and subsequently correct for electron-electron repulsion, referred to also as electronic correlation. As these methods are pushed to the limit, they approach the exact solution of the non-relativistic ScarГ¶dinner equation. In order to obtain exact agreement with experiment, it is necessary to include relativistic and spin orbit terms, both of which are only really important for heavy atoms. In all of these approaches, in addition to the choice of method, it is necessary to choose a basis set.
This is a set of functions, usually centered on the different atoms in the molecule, which are used to expand the molecular orbital with the ALCOA instant. ABA monition methods need to define a level of theory (the method) and basis set. The Heartier-Bock wave function is a single configuration or determinant. In some cases, particularly for bond breaking processes, this is quite inadequate, and and the coefficients of the basis functions are optimized together. The total molecular energy can be evaluated as a function of the molecular geometry; in other words, the potential energy surface.
Such a surface can be used for reaction dynamics. The stationary points of the surface lead to predictions of different isomers and the transition structures for conversion between isomers, but hose can be determined without a full knowledge of the complete surface. A particularly important objective, called computational thermometric, is to calculate thermoelectric quantities such as the enthalpy of formation to chemical accuracy. Chemical accuracy is the accuracy required to make realistic chemical predictions and is generally considered to be 1 kcal/mol or 4 k/mol.
To reach that accuracy in an economic way it is necessary to use a series of post-Heartier-Bock methods and combine the results. These methods are called quantum chemistry composite methods. Density functional methods Main article: Density functional theory Density functional theory (DAFT) methods are often considered to be ABA monition methods for determining the molecular electronic structure, even though many of the most communicational use parameters derived from empirical data, or from more complex calculations. In DAFT, the total energy is expressed in terms of the total one-electron density rather than the wave function.
In this type of calculation, there is an approximate Hamiltonian and an approximate expression for the total electron density. DAFT methods can be very accurate for little computational cost. Some ethos combine the density functional exchange functional with the Heartier-Bock exchange term and are known as hybrid functionalists. Semi-empirical and empirical methods Main article: Semi-empirical quantum chemistry methods Semi-empirical quantum chemistry methods are based on the Heartier- Bock formalism, but make many approximations and obtain some parameters from empirical data.
They are very important in computational chemistry for treating large molecules where the full Heartier-Bock method without the approximations is too expensive. The use of empirical parameters appears to allow some inclusion of relation effects into the methods. Semi-empirical methods follow what are often called empirical methods, where the two-electron part of the Hamiltonian is not explicitly included. For II-electron systems, this was the HГјcell method proposed by Erich HГјcell, and for all valence electron systems, the extended HГјcell method proposed by Royal Hoffmann.
Molecular mechanics Main article: Molecular mechanics In many cases, large molecular systems can be modeled successfully while avoiding quantum mechanical calculations entirely. Molecular mechanics simulations, for example, use a single classical expression for the energy of a compound, for instance the harmonic oscillator. All constants appearing in the equations must be obtained beforehand from experimental data or ABA monition calculations. The database of compounds used for parameterization, I. E. The resulting set of parameters and functions is called the force field, is crucial to the success of molecular mechanics calculations. A force field parameterized against a specific class of molecules, for other molecules of the same class. These methods can be applied to proteins and there large biological molecules, and allow studies of the approach and interaction (docking) of potential drug molecules (e. G. [l]and ). Methods for solids Main article: Computational chemical methods in solid state physics Computational chemical methods can be applied to solid state physics problems.
The electronic structure of a crystal is in general described by a band structure, which defines the energies of electron orbital for each point in the Brillion zone. ABA monition and semi- empirical calculations yield orbital energies; therefore, they can be applied to band Truckee calculations. Since it is time-consuming to calculate the energy for a molecule, it is even more time-consuming to calculate them for the entire list of points in the Brillion zone.
Chemical dynamics Once the electronic and nuclear variables are separated (within the Born- Oppenheim representation), in the time-dependent approach, the wave packet corresponding to the nuclear degrees of freedom is propagated via the time evolution operator (physics) associated to the time-dependent ScarГ¶dinner equation (for the full molecular Hamiltonian). In documentary’s energy- pendent approach, the time-independent ScarГ¶dinner equation is solved using the scattering theory formalism. The potential representing the interaction interaction is given by the potential energy surfaces.
In general, the potential energy surfaces are coupled via the vibration coupling terms. The most popular methods for propagating the wave packet associated to the molecular geometry are: * the split operator technique, * the Cohesive (real) polynomial, * the multi-configuration time-dependent Heartier method (MINCH), * the semicircular method. Molecular dynamics Main article: Molecular dynamics Molecular dynamics (MD) use either quantum mechanics, Newton’s laws of motion or a mixed model to examine the time-dependent behavior of systems, including vibrations or Brownian motion and reactions.
MD combined with density functional theory leads to hybrid models. Interpreting molecular wave functions The Atoms in molecules or QATAR model of Richard Bade was developed in order to effectively link the quantum mechanical picture of a molecule, as an electronic habituation, to chemically useful concepts such as atoms in molecules, functional groups, bonding, the theory of Lewis pairs and the valence bond model. Bade has demonstrated that these empirically useful chemistry concepts can be related to the topology of the observable charge density distribution, whether measured or calculated from a quantum mechanical habituation.