  Methods of determining surface diffusion coefficients of molecules from
signal fluctuations of a locally fixed probe are revisited and refined.
Particular emphasis is put on the influence of the molecule's extent. In
addition to the formerly introduced autocorrelation method and residence time
method, we develop a further method based on the distribution of intervals
between successive peaks in the signal. The theoretical findings are applied to
STM measurements of copper phthalocyanine (CuPc) molecules on the Ag(100)
surface. We discuss advantages and disadvantages of each method and suggest a
combination to obtain accurate results for diffusion coefficients.
