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tokens: Lind ( 2017 ) predict for Al a correction for dwarf and giant stars of ∼−0.1 dex at solar metallicity for the line at 669.6 nm. For Mg, we do not expect NLTE effects to have a strong influence on the abundances. Zhang et al. ( 2017 ) calculate corrections of the order of ±0.01 dex for stellar parameters similar to our NGC 6633 sample. For the same stars, Zhang et al. ( 2016 ) predict corrections of the order of −0.03 (RGB) and 0.00 dex (MS) for Si. The latter, however, has , i.e. the offset is relatively large and cannot be explained by NLTE effects. Since also in the case of M67, we found that the offset in Si between the upper MS and RGB is larger than predicted by the models, this could mean that indeed for Si there is a bias in the analysis of dwarf and giant stars. 5.4 APOGEE DR14 The recent fourteenth data release of the Apache Point Observatory Galactic Evolution Experiment (hereafter APOGEE DR14; see Abolfathi et al. 2017 ; Majewski et al. 2017 ) contains very important results concerning the abundances of M67 stars. In APOGEE DR14, stars in every evolutionary stage of M67 have been observed and analysed, from the MS to the RC. We decided to be as conservative as possible in the choice of the stars to be taken into consideration and we selected only those with STARFLAG, ASPCAPFLAG, and the ELEMFLAG of the element under study equal to zero. By doing so, we aim at excluding as many possible biases due to unreliable results as possible. APOGEE DR14 contains the abundances determined by the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) (see Holtzman et al. 2015 ; García Pérez et al. 2016 ) as well as calibrated abundances. These are calculated under the assumption that abundances of open cluster members should be homogeneous and that therefore any trend with temperature must be corrected for. Since for our purposes it is important to take into consideration any trend of the chemical abundances as a function of temperature, we decided to use the uncalibrated abundances. We present the results in the form [X/H]. For the elements that were calculated as [X/M], we add the uncalibrated [M/H] of the star. The error bars only represent the errors on the fit of the uncalibrated abundances presented in the APOGEE DR14 catalogue (for the uncalibrated [M/H] no errors are given). These are very small and are not to be considered realistic errors. Fig. 5 shows the CMD of the sample (not corrected for reddening) selected after a membership analysis based on radial velocity, PM, photometry and metallicity, and after all stars with the flags listed above not equal to zero were excluded. The stars are colour coded based on their iron abundance. It is already clear from this plot that [Fe/H] on the TO is lower than on the RGB and on the RC. Since no calibrated log g is available for dwarf stars in APOGEE DR14, we plotted in Fig. 6 the abundances of several species as a function of the effective temperature together with the models