After the first level of pseudoknot construction, free energy values are evaluated for each core H-type pseudoknot, which allows to filter unlikely pseudoknots. Only pseudoknots with low free energy (and therefore likely to form) will remain for the next step, which involves recursive secondary structure formation in the three pseudoknot loops. For all core H-type pseudoknots p 1 , . . . , p n , free energy ÁG is calculated as