Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, USA.
J Phys Chem A. 2010 Mar 4;114(8):2930-8. doi: 10.1021/jp911009f.
Spin unrestriction is typically defined as a free variation of the molecular orbitals of different spins in order to lower the molecular energy. When applied to approximate active space electron correlation methods, such as generalized valence bond coupled cluster (GVB-CC) models, this approach often leads to undesirable artifacts. We therefore present an alternative unrestricted in active pairs (UAP) procedure for spin polarization in GVB-CC methods, which resembles the corresponding orbitals of unrestricted Hartree-Fock theory. The UAP procedure permits spin polarization only within the two orbital subspaces describing each electron pair (consisting of one nominally occupied and one virtual orbital). This great reduction to just a linear number of degrees of freedom associated with spin polarization eliminates many of the undesirable artifacts associated with unconstrained spin unrestriction. The UAP procedure is tested on a variety of potential curves for bond breaking and the properties of small radicals as well as larger polyenyl radicals, allyl, pentadienyl, and heptatrienyl.
自旋无限制通常被定义为不同自旋的分子轨道的自由变化，以降低分子能量。当应用于近似活性空间电子相关方法时，例如广义价键耦合簇（GVB-CC）模型，这种方法通常会导致不理想的伪影。因此，我们提出了一种用于 GVB-CC 方法中自旋极化的替代活性对无限制（UAP）程序，该程序类似于无限制哈特ree-fock 理论的相应轨道。UAP 程序仅允许在描述每个电子对的两个轨道子空间内进行自旋极化（由一个名义占据轨道和一个虚拟轨道组成）。这种仅与自旋极化相关的自由度的大幅减少到线性数量，消除了许多与无约束自旋无限制相关的不理想伪影。UAP 程序在各种键断裂的势曲线以及小自由基和较大多烯基自由基、烯丙基、戊二烯基和庚三烯基的性质上进行了测试。
