Computational+Chemistry

Chemists have been some of the most active and innovative participants in this rapid expansion of computational science. Computational chemistry is simply the application of chemical, mathematical and computing skills to the solution of interesting chemical problems. It uses computers to generate information such as properties of molecules or simulated experimental results. Some common computer software used for computational chemistry includes: - History: Building on the founding discoveries and theories in the history of quantum mechanics, the first theoretical calculations in chemistry were those of Walter Heitler and Fritz London in 1927. The books that were influential in the early development of computational quantum chemistry include Linus Pauling and E. Bright Wilson's 1935 //Introduction to Quantum Mechanics – with Applications to Chemistry//, Eyring, Walter and Kimball's 1944 //Quantum Chemistry//, Heitler's 1945 //Elementary Wave Mechanics – with Applications to Quantum Chemistry//, and later Coulson'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 1940s, the solutions of elaborate wave equations for complex atomic systems began to be a realizable objective. In the early 1950s, 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 Sutcliffe. The first //ab initio// Hartree–Fock calculations on diatomic molecules were carried out in 1956 at MIT, using a basis set of Slater orbitals. For diatomic molecules, a systematic study using a minimum basis set and the first calculation with a larger basis set were published by Ransil and Nesbet respectively in 1960 The first polyatomic calculations using Gaussian orbitals were carried out in the late 1950s. The first configuration interaction calculations were carried out in Cambridge on the EDSAC computer in the 1950s using Gaussian orbitals by Boys and coworkers. By 1971, when a bibliography of //ab initio// calculations was published, the largest molecules included were naphthalene and azulene. Abstracts of many earlier developments in //ab initio// theory have been published by Schaefer.
 * Gaussian xx, Gaussian 94 currently
 * GAMESS
 * MOPAC
 * Spartan
 * Sybyl

In 1964, Hückel method calculations (using a simple linear combination of atomic orbitals (LCAO) method for the determination of electron energies of molecular orbitals of π electrons in conjugated hydrocarbon systems) of molecules ranging in complexity from butadiene and benzene to ovalene, were generated on computers at Berkeley and Oxford. These empirical methods were replaced in the 1960s by semi-empirical methods such as CNDO.

In the early 1970s, efficient //ab initio// computer programs such as ATMOL, GAUSSIAN, IBMOL, and POLYAYTOM, began to be used to speed up //ab initio// calculations of molecular orbitals. Of these four programs, only 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 MM2, were developed, primarily by Norman Allinger.

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 Fernbach and Abraham Haskell Taub, where they state "It seems, therefore, that 'computational chemistry' can finally be more and more of a reality." During the 1970s, 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.


 * electronic structure determinations
 * geometry optimizations
 * frequency calculations
 * transition structures
 * protein calculations, i.e. docking
 * electron and charge distributions
 * potential energy surfaces (PES)
 * rate constants for chemical reactions (kinetics)
 * thermodynamic calculations- heat of reactions, energy of activation

Currently, there are two ways to approach chemistry problems: **computational quantum chemistry** and **non-computational quantum chemistry** Computational quantum chemistry is primarily concerned with the numerical computation of molecular electronic structures by //ab initio// and semi-empirical techniques and non-computational quantum chemistry deals with the formulation of analytical expressions for the properties of molecules and their reactions. We just mentioned //ab initio// and semi-empirical numerical techniques. Definitions of these terms are helpful in understanding the use of computational techniques for chemistry. Scientists mainly use three different methods to make calculations:


 * //ab initio//, (Latin for "from scratch") a group of methods in which molecular structures can be calculated using nothing but the Schroedinger equation, the values of the fundamental constants and the atomic numbers of the atoms present (Atkins, 1991).
 * Semi-empirical techniques use approximations from empirical (experimental) data to provide the input into the mathematical models.
 * Molecular mechanics uses classical physics to explain and interpret the behavior of atoms and molecules

To summarize, computational chemistry is:
 * a branch of chemistry that generates data which complements experimental data on the structures, properties and reactions of substances. The calculations are based primarily on Schroedinger's equation and include:
 * 1) calculation of electron and charge distributions
 * 2) molecular geometry in ground and excited states
 * 3) potential energy surfaces
 * 4) rate constants for elementary reactions
 * 5) details of the dynamics of molecular collisions
 * particularly useful for:
 * 1) determination of properties that are inaccessible experimentally
 * 2) interpretation of experimental data