  The combination of high-contrast imaging and medium to high spectral
resolution spectroscopy offers new possibilities for the detection and
characterization of exoplanets. The molecular mapping technique uses the
difference between the planetary and stellar spectra. While traditional
post-processing techniques are quickly limited by speckle noise at short
angular separation, it efficiently suppresses speckles. Its performance depends
on multiple parameters such as the star magnitude, the adaptive optics residual
halo, the companion spectrum, the telluric absorption, as well as the telescope
and instrument properties. Exploring this parameter space through end-to-end
simulations to predict potential science cases and to optimize future
instrument designs is very time-consuming, making it difficult to draw
conclusions. We propose to define an efficient methodology for such an
analysis. Explicit expressions of the estimates of signal and noise are
derived, and they are validated through comparisons with end-to-end
simulations. They provide an understanding of the instrumental dependencies,
and help to discuss optimal instrumental choices with regard to the targets of
interest. They are applied in the case of ELT/HARMONI, as a tool to predict the
contrast performance in various observational cases. We confirm the potential
of molecular mapping for high-contrast detections, especially for cool planets
at short separations. We provide guidelines based on quantified estimates for
design trade-offs of future instruments. We discuss the planet detection
performances of HARMONI observing modes. While they nicely cover the
appropriate requirements for high detection capability of warm exoplanets, a
transmission extended down to J band would be beneficial. A contrast of a few
1E-7 at 50mas should be within reach on bright targets in photon noise regime
with molecular mapping.
