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Multiple choice
In the single-excitation subspace {∣01⟩,∣10⟩}\{|01\rangle, |10\rangle\}, an H2 Hamiltonian gives the 2x2 block [accb]\begin{bmatrix}a & c \\ c & b\end{bmatrix} with a=+0.2066a = +0.2066, b=−1.3498b = -1.3498, and off-diagonal coupling c=g4+g5=0.0910+0.0910=0.182c = g_4 + g_5 = 0.0910 + 0.0910 = 0.182. The ground-state energy is (a+b)/2−((a−b)/2)2+c2(a+b)/2 - \sqrt{((a-b)/2)^2 + c^2}. What is it (Hartree)?