CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

potential energy surface for these clusters and finding a minimum on the surface. The key
issue in this search procedure is to guess a good initial geometry of the cluster, which
might converge during the calculation to a local or the global minimum. It is to be noted
that the adopted procedure for geometry search cannot guarantee to locate the global
minimum. Thus, Monte Carlo based simulated annealing procedures have also been
applied with the effective fragment solvent molecules to find out the global minimum
structure of different size molecular clusters. 36 In this method, molecular coordinates are
displaced by a random amount and the energy of the system has been evaluated at the
new structure. The new structure with lower energy is accepted while structures with
higher energies have been accepted at a probability determined by the Boltzmann factor.
Random structures are generated by carrying out Monte-Carlo steps at a temperature
range of 2000 K to 100 K for more than 8000 steps. 36
In molecular electronic structure methods the energy of a system is calculated
using the self consistent field (SCF) method. For calculation of the energy system many
methods are available. Among them the simplest ab initio method is Hartree-Fock (HF),
which is an electron uncorrelated method.
In all the SCF method the density matrix (P) is guessed initially and is defined for
restricted HF case 37
occupied
P λσ
2 χ λ iχ
σ i
i
= ∑ (5)
where the coefficient ( χ ) comes from the linear combination of the basis set ( ϕ
i
)
N
φ = ∑ χϕ
(6)
j ij i
i=
1
11