  A new functional form for the exchange enhancement in the generalized
gradient approximation within density functional theory is given. The
functional form satisfies the constraints used to construct the
Perdew-Burke-Ernzerhof (PBE) functional but can be systematically varied using
one parameter. This gives the possibility to estimate the reliability of a
computational result or to fit the parameter for a certain problem. Compared to
other semi-empirical functionals, the present has the advantage that only one
physically transparent parameter is used and that the fitted functional will
obey the same exact conditions as PBE functional. Furthermore the simple form
of the exchange enhancement means that oscillating terms in the exchange
potential are avoided.
