  Recently, Song et al [Nature 462, 1039 (2009)] employed transition-voltage
spectroscopy to demonstrate that the energy $\varepsilon_H$ of the highest
occupied molecular orbital (HOMO) of single-molecule transistors can be
controlled by a gate potential $V_G$. To demonstrate the linear dependence
$\varepsilon_H-V_G$, the experimental data have been interpreted by modeling
the molecule as an energy barrier spanning the spatial source-drain region of
molecular junctions. Since, as shown in this work, that crude model cannot
quantitatively describe the measured $I$-$V$-characteristics, it is important
to get further support for the linear dependence of $\varepsilon_H$ on $V_G$.
The results presented here, which have been obtained within a model of a
point-like molecule, confirm this linear dependence. Because the two models
rely upon complementary descriptions, the present results indicate that the
interpretation of the experimental results as evidence for a gate controlled
HOMO is sufficiently general.
