Abstract We investigated the ground state of approximants consisting of ⩽ 165 Si atoms ( d QD ⩽ 18.5 A ) with full termination of the Si interface with F, OH, NH2, CH3 and H groups simulating Si QDs embedded in an ionic environment, SiO2, Si3N4, SiC matrix and a co-valent environment, respectively, with ab initio methods. As the polarity of the Si/matrix interface increases the optical band-gap becomes increasingly dominated by charge transfer at the interface rather than by quantum confinement. For Si QDs with d QD = 7.3 –37 A, the interface determines the electronic structure in competition with quantum confinement for H and CH3 terminations and only as a secondary effect for strong polar interfaces (NH2, OH). We present an estimate of band gaps of different QD materials with the same interface and interpret the ab initio results in conventional quantum mechanics.