  Ground state of two-electron quantum dots in single-valley materials like
GaAs is always a spin singlet regardless of what the potential and interactions
are.
  This statement cannot be generalized to the multi-valley materials like
$n$-doped Si.
  Here we calculate numerically the spectrum of a two-electron Si quantum dot
and show that the dot with the lateral size of several nm can have the spin
triplet ground state which is impossible in the single-valley materials.
  Predicted singlet-triplet level crossing in two-electron Si quantum dots can
potentially establish the platform for quantum simulation of magnetic many body
systems based on quantum dots.
  We suggest several examples of such systems that open a way to controlled
quantum simulations within the condensed matter setting.
