The obvious thing that would pop up in one’s mind after reading the title of the article is that it involves computers; some may also refer to computer programs. All of them are absolutely right. In fact, the name of the subject under which the topic falls has a touch of “computer”, it’s computational chemistry. It is that branch of chemistry which uses computer simulations to assist in solving chemical problems.[1] Computer simulation is a technique wherein a system is represented by a mathematical model and the outcomes of the model are simulated using computers and computer programs. Initially, there were two pillars of science, namely – theory and experiments. In theory, one makes predictions and/or proposes hypotheses which can be either accepted or rejected upon verification via experiments. In most of the cases, experiments help us narrow down the number of possible theories by rejecting the incorrect ones. The rapidly emerging third pillar, simulations, has significantly contributed to the volume of literature in the past few decades. It is a very powerful tool for analysis. It is used, mostly, when the experiments are too hard, too dangerous or too expensive to carry out. Apart from these, the most important use of simulation is to help identify feasible chemical/physical processes among numerous possibilities and plan the physical experiment accordingly. Theory births a conceptual idea giving rise to complicated equations which, at times, are very difficult to solve analytically, are time consuming or yield complicated solutions that are very difficult to interpret or imagine. Therefore, scientists have introduced computer simulations to facilitate the process. In a nutshell, theories generate equations; simulations provide solutions to those equations which subsequently helps in interpreting the experimental results. Experimental data is required to further fine-tune the parameters of the simulation so as to make more accurate predictions of the behaviour of a system. To run a simulation over a physical/chemical system, one should first define a model representing that system. Technically, a model can be defined as a simplified mathematical representation of a system using which one can easily understand its properties, functions and underlying mechanisms for the aforementioned functions. For example, the ball and stick model of molecules is used in understanding their geometry; the mathematical model of wave functions helps understand how they interact with one another, how the orbitals overlap leading to the formation of chemical bonds. In computational chemistry, we attempt to find the solutions for the concerned numerical equations, enabling the qualitative and/or quantitative predictions of the properties of a system. The main strategy involves developing concepts and building up a model to explain observed properties and facts:

Make a model of the system of interest by using an appropriate mathematical description and approximations

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Choose suitable parameters for the model

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Perform computer simulations

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Plan out experiments (if possible) to check the reliability of the model by verifying the outcomes of predictions.

The applications of computational chemistry are in exploring reaction mechanisms (e.g., for organic and inorganic synthesis), studying complex structures and interactions, designing new molecular structures,identifying promising systems or processes etc. In a practical sense, this covers a vast area of interdisciplinary research involving chemistry, physics, material sciences, chemical engineering and biology.[2] Employing computational chemistry or a computer simulation technique, electronic-structure, molecular dynamics in condensed phases, proton transfer process in biological reactions, pharmaceutical chemistry (development of medicines), homogeneous catalysis and a variety of other physical and chemical events can be studied. One of the most recent and emergent areas of research in this field is quantum simulation. These simulations use quantum effects to describe model systems from which the description can be extended to real systems. Despite the use of powerful computers to carry out such quantum simulations, we need quantum simulators.[3] Many researches are involved in developing these simulators, whose main aim is to make more realistic predictions in the shortest time possible. The books ‘Introduction to Quantum Chemistry-with Applications to Chemistry’ by Linus Pauling and Edgar Bright Wilson in 1935, ‘Quantum Chemistry’ by Eyring, Walter and Kimball in 1944, ‘Elementary Wave Mechanics with Applications to Quantum Chemistry’ by Heitler in 1945 and later ‘Valence’ by Coulson in 1952 laid the foundations for computational chemistry. Then with the advancements in the field of computer science, the use of computer programs or simulations, analysis of complex molecules became easier. In the 1950’s, the first semi-empirical atomic orbitals were performed using a computer. The term computational chemistry was first mentioned in the book ‘Computers and Their Role in the Physical Sciences’ by Sidney Fernbach and Abraham Haskell Taub. In their book, they stated that “It seems, therefore, that ‘computational chemistry’ can finally be more and more of a reality”

References

[1] C. J. Cramer, Essentials of Computational Chemistry, 2nd edition, John Wiley & Sons (2004).

[2] S. Paulo, Chemistry International, 2017, 2, 39, 43-44.

[3] Johnson et al., E P J Quantum Technology, 2014, 1, 10.