The activity parameters for molecules, which are mainly used in molecular catalysis, design of biologically active substances, etc., are somewhat different. First and foremost in this list of parameters are the values of ionization potential (IP) and electron affinity (EA) and the associated energies of the highest occupied molecular orbital (HOMO) IP=-ε HOMO and the lowest unoccupied molecular orbital (LUMO) EA=-ε LUMO . There are also a number of derived parameters based on HOMO and LUMO: orbital energy difference (ε HOMO-LUMO ), chemical potential (µ), hardness (η), softness (S), electronegativity (χ), and global electrophility index (ω). These molecular descriptors find applications in catalysis, corrosion studies, evaluation of biological activity of compounds, etc. In addition, complex descriptors for neutral molecules and ions were proposed for characterizing some parameters of complexes (dissociation constants, reaction rate constants, etc.). These parameters describe specific properties of complexes of molecules or ions based on a whole range of parameters of the system under study (dipole moment, molecular volume, hydrogen bonding strength, etc.).