Zgid Dominika, Nooijen Marcel
Department of Chemistry, University of Waterloo, 200 University Ave. West, Waterloo N2L 3G1, Canada.
J Chem Phys. 2008 Jan 7;128(1):014107. doi: 10.1063/1.2814150.
We present a spin-adapted density matrix renormalization group (DMRG) algorithm designed to target spin and spatial symmetry states that can be difficult to obtain while using a non-spin-adapted algorithm. The algorithmic modifications that have to be introduced into the usual density matrix renormalization group scheme in order to spin adapt it are discussed, and it is demonstrated that the introduced modifications do not change the overall scaling of the method. The new approach is tested on HNCO, a model system, that has a singlet-triplet curve crossing between states of the same symmetry. The advantages of the spin-adapted DMRG scheme are discussed, and it is concluded that the spin-adapted DMRG method converges better in almost all cases and gives more parallel curves to the full configuration interaction result than the non-spin-adapted method. It is shown that the spin-adapted DMRG energies can be lower than the ones obtained from the non-spin-adapted scheme. Such a counterintuitive result is explained by noting that the spin-adapted method is not a special case of the non-spin-adapted one; consequently, the spin-adapted result is not an upper bound for the non-spin-adapted energy.
我们提出了一种自旋适配的密度矩阵重整化群（DMRG）算法，该算法旨在针对使用非自旋适配算法时可能难以获得的自旋和空间对称态。讨论了为使通常的密度矩阵重整化群方案实现自旋适配而必须引入的算法修改，并证明所引入的修改不会改变该方法的整体缩放比例。在模型系统HNCO上对新方法进行了测试，该系统在相同对称态之间存在单重态 - 三重态曲线交叉。讨论了自旋适配DMRG方案的优点，并得出结论，自旋适配DMRG方法在几乎所有情况下收敛性更好，并且与非自旋适配方法相比，能给出更多与完全组态相互作用结果平行的曲线。结果表明，自旋适配DMRG能量可能低于非自旋适配方案得到的能量。通过指出自旋适配方法不是非自旋适配方法的特殊情况来解释这种违反直觉的结果；因此，自旋适配结果不是非自旋适配能量的上限。
