Ligand efficiency is a measurement the binding energy

Binding energy

Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...

 per atom of a ligand

Ligand

In coordination chemistry, a ligand is an ion or molecule that binds to a central metal atom to form a coordination complex. The bonding between metal and ligand generally involves formal donation of one or more of the ligand's electron pairs. The nature of metal-ligand bonding can range from...

 to its binding partner, such as a receptor or enzyme.

Ligand efficiency is used in drug discovery

Drug discovery

In the fields of medicine, biotechnology and pharmacology, drug discovery is the process by which drugs are discovered or designed.In the past most drugs have been discovered either by identifying the active ingredient from traditional remedies or by serendipitous discovery...

 research programs to assist in narrowing focus to lead compound

Lead compound

A lead compound in drug discovery is a chemical compound that has pharmacological or biological activity and whose chemical structure is used as a starting point for chemical modifications in order to improve potency, selectivity, or pharmacokinetic parameters.Lead compounds are often found in...

s with optimal combinations of physicochemical properties and pharmacological properties.

Mathematically, ligand efficiency (LE) can be defined as the ratio of Gibbs free energy

Gibbs free energy

In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...

(ΔG) to the number of non-hydrogen atoms of the compound:

LE = (ΔG)/N

where ΔG = -RTlnK_i and N is the number of non-hydrogen atoms.