  We propose a method to evaluate the Gibbs free energy from constant-volume
first-principles calculations. The volume integral of the pressure is performed
by determining the volume and the bulk modulus in equilibrium at finite
temperatures, where the pressure and its volume derivative are evaluated
utilizing first-principles calculations of the Gr\"uneisen parameter without
varying the volume. As an example, the validity of our method is demonstrated
for fcc-Al by comparing with the conventional quasiharmonic approximation that
is much more computationally-demanding.
