CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

Boyd proposed the use of Gaussian type orbitals (GTOs). The general functional form of
a GTO in atom-centered Cartesian coordinate is,
ϕ i
Ax y z e α
2 2 2
i j k − ( x + y + z )
= (14)
where A is the normalization constant, α is an exponent controlling the width of the
GTO, and i, j, and k are non-negative integers that indicate the nature of the orbital in a
Cartesian sense. 37 As the GTOs do not have proper ‘cusp’ at the nuclear centre, Boyd
suggested that the combination of several GTOs with fixed combination coefficients will
mimic the STOs. When a basis functions is defined as a linear combination of GTOs , is
known as contracted GTOs (CGTOs) and the individual GTOs from which it is formed is
known as primitive Gaussians. A CGTOs of the from 6-311+G(d) means that 6
primitives for the core, 3, 1, 1 primitives for the triply split valence orbital and the diffuse
and polarizations functions are represented by + and d symbol, respectively. For these
type of CGTOs the exponent and contraction coefficients are determined by uncorrelated
HF method. When the exponent and contraction coefficients are variationally optimized
not only for HF but also for the electron correlated method, the basis is known as
‘correlated consistent polarized valence’ (cc-pVDZ, cc-pVTZ etc) basis set. If they are
augmented with extra set of diffuse functions, they are called as aug-cc-pVDZ, etc.
1.4. Scope of the Present Thesis
To understand the process of microsolvation, a detailed study on the structure and
energetics of finite size cluster of various chemical species in water, ammonia and carbon
dioxide solvents have been carried out. Both close and open shell chemical species are
considered for investigation. At present halogen dimer radical anions (Cl •− 2 , Br •− 2 & I •− 2 ),
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