A few level approach for the electronic partition function of atomic systems

A simplified model to calculate partition functions and thermodynamic properties of atomic species is presented. This model consists in grouping the atomic states in few virtual levels. Their statistical weights and energies are calculated summing or mediating over the states belonging to each group. The partition function is calculated considering the virtual levels to follow the Boltzmann distribution. A theoretical foundation of the model has been described for a general case and verified for hydrogen, oxygen and nitrogen atoms. Two- and three-level models are adequate to keep the error within a few percent. A second order correction term can also be added to further reduce the error without changing the advantages of the model.