Basis-set extrapolation-a way to achieve the best possible

For reliable results in the scattering phenomenon, accurate interaction potential is required.1–7 The interaction potential may be calculated with the help of MOLPRO (Werner et al., 2012), for example, where one has to use the best available method and the best available basis-set. The best method available is the couple dcluster with Single and Double, and perturvative Triple excitations CCSD(T).8 The best basis-set available may be the aug-cc-pVnZ, where n denotes the number of zeta used and it stands for one of the D (Double-zeta), T (Triple-zeta), Q (Quadruple-zeta), 5 (Quintuple-zeta), etc. The prefix ’aug’ denotes the augmented version of the basis set. With the increase of n, the accuracy increases, and one would like to have n=∞. But, the requirement of memory size and computer-time increases extremely fast with the increase of n, which is not affordable, beyond n=6 (say). Truhlar9 proposed a nice concept for extrapolation of basis-set, where total energy is expressed as the sum of the reference energy (HF) and the energy of correlations.