  There are worldwide efforts to maximize the energy saving achieved by
VO$_2$-based thermochromic coatings. In particular, there are very different
values of the modulation of integral solar energy transmittance, reported by
various laboratories on various templates even for seemingly very similar
coatings. A detailed analysis reveals that this is largely due to the
combination of the intentional and well understood transmittance modulation in
the infrared (always beneficial) with not yet understood slight transmittance
modulation in the visible (sometimes beneficial, sometimes harmful, and always
multiplied by strong solar irradiance). Ab-initio calculations are used to
examine the hypothesis that the transmittance modulation in the visible can be
controlled by lattice strain and/or slightly off-stoichiometric [O]/[V] ratio.
The presented phenomenon opens a pathway which may lead, in a case of
reproducible preparation of correctly altered VO$_2$, to significantly enhanced
energy saving.
