ABSTRACT We study the initial conditions of a common envelope (CE) event resulting in a stellar merger. A merger’s dynamics could be understood through its light curve, but no synthetic light curve has yet been created for the full evolution. Using the smoothed particle hydrodynamics (SPH) code StarSmasher, we have created three-dimensional (3D) models of a 1.52 M⊙ star that is a plausible donor in the V1309 Sco progenitor. The integrated total energy profiles of our 3D models match their initial one-dimensional (1D) models to within a 0.1 per cent difference in the top 0.1 M⊙ of their envelopes. We have introduced a new method for obtaining radiative flux by linking intrinsically optically thick SPH particles to a single stellar envelope solution from a set of unique solutions. For the first time, we calculated our 3D models’ effective temperatures to within a few per cent of the initial 1D models, and found a corresponding improvement in luminosity by a factor of ≳106 compared to ray tracing. We let our highest resolution 3D model undergo Roche lobe overflow with a 0.16 M⊙ point-mass accretor (P ≃ 1.6 d) and found a bolometric magnitude variability amplitude of ∼0.3 – comparable to that of the V1309 Sco progenitor. Our 3D models are, in the top 0.1 M⊙ of the envelope and in terms of total energy, the most accurate models so far of the V1309 Sco donor star. A dynamical simulation that uses the initial conditions we presented in this paper can be used to create the first ever synthetic CE evolution light curve.