Abstract: This chapter first discusses the difference between semi-empirical and ab initio methods, then devotes most of the remaining pages to ab initio calculations. Each ab initio calculation is defined by two ‘parameters’: the basis set employed and the level of electron correlation adopted. These two topics are discussed in some detail. Density functional theory is also discussed, which has gained popularity in recent years. After describing these computational methods, a brief assessment on the performance of various levels of theory in yielding structural parameters, vibrational frequencies, and energetic quantities is given. Finally, a few examples are selected from recent literature to show how computations complement experiments to arrive at meaningful conclusions.

Keywords: ab initio methods, basis set, configuration interaction, correlation energy, coupled cluster method, density functional theory, diffuse function, minimal basis set, polarization function, semi-empirical methods,