# 04 – Energia de ligação

We can use the total energy calculated by DFT to determine other physical and chemical properties. A very useful and simple physical property to calculate is the binding energy. To calculate this, we need at least 3 simulations. A simulation of the composition of two subsystems (A + B), and a simulation for each separate subsystem. Thus, we define the binding energy between two subsystems A and B as:

where E_{tot}(X) is the total energy calculated by the DFT of system X. This energy is defined so that if the value is negative there is an attraction between subsystems A and B. If it is positive there is a repulsion between A and B. If one of the subsystems is a surface, the binding energy is called adsorption energy.

### EXEMPLO 04 (Sistema Lítio-Coroneno)

As an example let’s calculate the binding energy between a Li atom and a coronene molecule. This coronene molecule is a polycyclic aromatic hydrocarbon (PAH) formed by the junction of 7 benzene rings as shown in Figure XX. As mentioned earlier, this is accomplished through 3 simulations.

#### Lithium+Coronene simulation

The atomic coordinates of this composite system can be viewed in the *xyz* file of example05 in our github repository.