We have used the density functional ab initio method to conduct investigations on the effects of an applied electric field on the chemisorption bonds of adsorbate-surface systems, and on the reactivity of a gas phase semiconductor cluster. In STM current-induced excitation of adsorbates, lateral energy transfer among adsorbates tend to delocalize the excitation, and reduce resolution. We show that the strength of chemical bonds can be increased or decreased depending on the strength and direction of the applied electric field. By shifting the excitation energy of an adsorbate below the tip, energy transfer away from the site can be inhibited, and thereby lead to adsorbate excitation localization. The details of the field-induced shifts of an adsorbate-surface bonding features are shown to depend on their dipolar polarization. In the case of the reactivity of GaAs clusters with ammonia, recent experiments indicate that NH\sb3 adsorption rate depends strongly on cluster size, Ga/As composition ratio, and cluster charge state. We characterize the reactivity of NH\sb3 at various sites of a Ga\sb5As\sb5 cluster in terms of the adsorbate binding energy and charge transfer, showing a strong correlation between the two. The dependence of the cluster reactivity on its charge state is deduced.