CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

density functional, namely, Becke’s half-and-half (BHH) exchange and Lee-Yang-Parr
(LYP) correlation functional (BHHLYP). Split valence basis set with diffusion and
double polarization functions on all the atoms, 6-311++G(2d,2p) has been used in all
calculations. Pseudo Newton Raphson based algorithm has been applied to look for the
minimum energy structures for each K.nNH 3 cluster. The major concern in this algorithm
is to guess a good starting geometry of the cluster, which might converge during the
calculation to a local or the global minimum. In the present calculation, several possible
starting geometries are generated based on different possible three-dimensional
structures. Hessian calculations have been performed to check the nature of the optimized
structures, to calculate thermodynamic parameters and to generate IR spectrum of the
clusters. In a number of cases, first hand optimized minimum energy structures of these
open shell systems have yielded one or more normal modes having imaginary frequency.
In these cases, the structures have been reoptimized focusing attention on these normal
modes till the structures generate no imaginary frequency. Population of the minimum
energy configurations of each size clusters has been calculated based on free energy
change (ΔG) at 150 K following Boltzmann population distribution.
6.3. Results and Discussion
6.3.1. Structure
Only one stable minimum energy structure is obtained through full geometry
optimizations of small size (n=1-3) K.nNH3 clusters. It is to be noted that the geometries
of clusters having different orientations of N atoms from NH 3 molecule with respect to K
atom are considered as the initial guess structures in the present study and only the most
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