MC-QCISD: Multi-coefficient correlation method based on quadratic configuration interaction with single and double excitations

This paper presents a multi-coefficient correlation method based on quadrat ic configuration interaction with single and double excitations (MC-QCISD) and basis sets using segmented contraction and having the same exponential parameters in the s and p spaces. The results are comparable to a previous multi-coefficient correlation method based on coupled cluster theory with l ess efficient correlation-consistent basis sets, and they are better than a previous multi-coefficient correlation method based on Moller-Plesset four th order perturbation theory with single, double, and quadruple excitations with correlation-consistent basis functions. The mean unsigned error per b ond of the MC-QCISD method is 0.72 kcal/mol. The new method should be very efficient for computing geometries of open-shell transition states.