(From AOganov 030207)
EOSFIT : very useful program for fitting P(V) equations of state.
http://www.crystal.vt.edu/crystal/software.html

EOS: very useful and usually robust program for fitting E(V) equations of state.
http://physig.uni-graz.at/~kde/eos.html
I would recommend to decrease steps in fitdata.f by ~10 times for high precision