Hardness, Softness and Other Descriptors

2 Hardness, Softness, and Other Descriptors. - When two molecules, A and B, interact, the way they interact is to a large extent often determined by properties of the isolated A and molecules. In order to quantify these properties, the hardness and softness of Pearson42 can be useful. These quantities were put on a rigorous formal foundation by Parr et al43 within density-functional theory and have since then be the subject of some theoretical studies. Here, we shall briefly review their definitions and subsequently consider the various quantities for some few systems. 

We analyse how the chemical potential changes when the system is modified, whereby the modifications can be due either to changes in the number of electrons or to changes in the (external) potential Feff. These changes can occur when the system of interest starts interacting with some other system. Consider- 

The first quantity on the right-hand side is related to the chemical hardness, 

This information can be very useful when studying the initial states of chemical reactions. One can imagine that two molecules interact and that the interaction involves some electron transfer between the two. Then, it is of advantage for the interaction that the two molecules are so placed relative to each other that the electron-accepting part easily can accommodate this extra electron density and that, equivalently, the electron-donating part easily can donate this density. 

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