\textit{Ab initio} studies on molecules and clusters in external electrostatic fields

Influence of a uniform static external electric field on some aliphatic and aromatic molecular species is studied within the density functional theory (DFT) employing the 6-311++G(2d,2p) basis-set with B3LYP exchange-correlation prescription. The electric field perturbs the molecular geometry; alters the dipole moments and engenders a molecular vibrational Stark effect. For polar molecules, significant frequency shifts are observed for field orientations both parallel and antiparallel to their permanent dipole moments; where HOMO-LUMO gaps alter significantly. Time dependent DFT analysis reveals that an increase in the applied field strength increases the excitation energies amongst frontier MOs with a concomitant decrease in oscillator strengths. Structural evolution of water clusters, (H$_{2}$O)$_{n}$, n=6-8 is studied within DFT: the intermolecular hydrogen bonds stretch, and eventually break at some threshold values, triggering a conformational transformation, with configurations appearing as local minima on the cluster's potential energy landscape, with abrupt increase in the electric dipole moments and `opening up' of three dimensional morphologies of water.