= Tetrahedral doublet distortion-sign ambiguity
{title2=$E(Q)=KQ^2/2-g|Q|$}
For a linearly split orbital doublet occupied by one <electron> or one <hole>, the lower adiabatic branch has a cusp $-g|Q|$. A harmonic elastic cost produces equal minima at $Q=\pm g/K$. Orbital degeneracy therefore establishes instability of the regular geometry, but cannot alone distinguish opposite tetragonal distortions. A symmetry-allowed cubic elastic invariant in the two-component distortion can select one sign.
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