Computational Chemistry: Semi-Empirical Methods And Ab initio methods

Difficulties arising from the assumption Huckel method has been overcome by theories more sophisticated, which not only calculates the shape and molecular orbital energy but also can fairly accurately predict the structure and reactivity of molecules. Full treatment electronic structure of molecules is quite easy to formulate but difficult to implement. However, it received a large amount of attention chemists, and has become the key of modern chemical research. John Pople and Walter Kohn was awarded the Nobel Prize for Chemistry in 1998 for his contribution to the development of computational techniques are explained strktur and reactivity of molecules.

Semi-Empirical Methods

Semi-empirical method is a type of mechanical quantum chemical calculations using parameters derived from experiment to simplify the calculation. In the semi-empirical method integral to interpret the many interesting spectroscopic data and physical properties such as ionization energy and uses a set of rules to regulate the definite integral equal to zero. Semi-empirical method can be applied in large systems and produce good wave functions so that the electronic properties can be predicted. semi-empirical method relies on the availability and reliability of empirical parameters rather low.

One simplistic approach is called complete neglect of differential overlap (CNDO), where all the integral is set to zero unless A and B are the same orbital centered on the same core, and also C and D. Integral live then adjusted up energy levels are within good agreement with experiment.

Ab initio methods

Ab initio theory is a concept that is common calculation of settlement prktis Schrodinger equation that can predict the accuracy and guilt. This method was made to calculate all integral that appears in the Fock and overlap matrix.

Fock matrix has elements comprising integral form:

where A, B, C, and D are atomic orbitals that can generally be centered on a different core. It can be understood that, if there are several dozen atomic orbitals used to construct the molecular orbitals, then there will be tens of thousands of these forms to evaluate the integral (integral number increases as the fourth power of the atomic orbitals numbers at the bottom). Ab initio methods requires the ability and speed of large computer.