Local softness, scanning tunneling microscopy, and surface reactivity

Within the finite temperature extension of density functional theory, it is shown that local softness measures the ability of a chemical system to gain or donate charge as the external potential changes. This feature allows a formal connection between local softness and scanning tunneling microscopy (STM) images to be established. Under appropriate conditions, STM images can be used to measure local softness for surfaces. A potential application of those images as reactivity criteria within the context of the hard and soft acids and bases (HSAB) principle is discussed.