H1 = 1 + 2sx
H2 = 2 + 4sz +3sx
H1H2 = 8 + 7sx + 4sz -i8sy
[H1,H2] = 8[sx,xz] = -i16sy
eig(H1) = 1 \pm 2
eig(H2) = 2 \pm 5
tr(H1H2) = tr(1*2+2*3) = 16