Kiel diagram showing the RSG pulsation cycle and the MARCS model coverage. Dots along the RSG track are spaced equally in time every 1/20 of the pulsation period. The crosses show the combinations of T_eff and log g for which there are MARCS models available and for which we can compute DUSTY+MARCS (DM) models. After extending the grid by one grid spacing in each direction (green squares), we can cover 80% of the RSG pulsation cycle by linearly interpolating the newly obtained grid of DM models (shaded region).