  Carpi (1993) and Lepisto (1994) proved independently that smooth words are
cube-free for the alphabet {1, 2}, but nothing is known on whether for the
other 2-letter alphabets, smooth words are k-power-free for some suitable
positive integer k. In this paper, we first establish the derivative formula of
the concatenation of two smooth words and power derivative formula of smooth
words over arbitrary 2-letter alphabets. Then by making use of power derivative
formula, for arbitrary 2-letter alphabet {a, b} with a, b being positive
integers and a<b, we prove that smooth words are (b+1)-power-free except for
a=1 and b=3; and smooth words are quintic-free and there are infinitely many
smooth biquadrates for the alphabet {1, 3}. Moreover, we give the number of
smooth words of form w^n with a and b having the same parity.
