  We performed extensive tests of the accuracy of atmospheric parameter
determination for FGK stars based on the spectrum fitting procedure
Spectroscopy Made Easy (SME). Our stellar sample consists of 13 objects,
including the Sun, in the temperature range 5000--6600~K and metallicity range
-1.4 -- +0.4. The analysed stars have the advantage of having parameters
derived by interferometry. For each star we use spectra obtained with different
spectrographs and different signal-to-noise ratios (S/N). For the fitting we
adopted three different sets of constraints and test how the derived parameters
depend upon the spectral regions (masks) used in SME. We developed and
implemented in SME a new method for estimating uncertainties in the resulting
parameters based on fitting residuals, partial derivatives, and data
uncertainties. For stars in the 5700--6600 K range the best agreement with the
effective temperatures derived by interferometry is achieved when spectrum
fitting includes the H$\alpha$ and H$\beta$ lines, while for cooler stars the
choice of the mask does not affect the results. The derived atmospheric
parameters do not strongly depend on spectral resolution and S/N of the
observations, while the uncertainties in temperature and surface gravity
increase with increasing effective temperature, with minima at 50~K in Teff and
0.1~dex in log g, for spectra with S/N=150--200. A NLTE analysis of the
TiI/TiII and FeI/FeII ionisation equilibria and abundances determined from the
atomic CI (NLTE) and molecular CH species supports the parameters we derived
with SME by fitting the observed spectra including the hydrogen lines.
