Syntax: mkteci file1.eci ...
where file1.eci etc. is a list of T-dependent eci files obtained with
the clusterexpand command, e.g.
clusterexpand fvib in the case of vibrational free energy or
clusterexpand felec in the case of electronic free energy.
Usually the file list is: fvib.eci felec.eci
^vibrational ^electronic free energy