We will first discuss the difference between the SD-based Stochastic-CASCI and the spinadapted Stochastic-CASCI spin-ladders. Figure shows that the CASCI spin ladder in SD basis is in good agreement with the spin-adapted CASCI results. Some marginal deviations appear for s ≤ 3. The largest deviation occurs for s = 2, and it is less than 1 kcal mol -1 .