Algorithms predicting RNA secondary structures based on different fold
ing criteria - minimum free energies (mfe), kinetic folding (kin), max
imum matching (mm)- and different parameter sets are studied systemati
cally. Two base pairing alphabets were used: the binary GC and the nat
ural four-letter AUGC alphabet. Computed structures and free energies
depend strongly on both the algorithm and the parameter set. Statistic
al properties, such as mean number of base pairs, mean numbers of stac
ks, mean loop sizes, etc., are much less sensitive to the choice of pa
rameter set and even of algorithm. Some features of RNA secondary stru
ctures, such as structure correlation functions, shape space covering
and neutral networks, seem to depend only on the base pairing logic (G
C or AUGC alphabet).