Department of Physics, Taki Government College, Taki, North 24 Parganas 743429, India.
J Chem Phys. 2010 Aug 21;133(7):074102. doi: 10.1063/1.3467899.
We have employed complete active space based single root multireference coupled cluster method (the resulting method is referred to by the acronym sr-MRCC) to compute the potential energy surfaces (PESs) of some well studied "protypical model" systems for which a highly accurate and reliable database is available for comparison. As that of state-specific theory, the sr-MRCC approach focuses and correlates one state while using a multiconfigurational reference and thus it naturally avoids intruder states. The present method is structurally different from the well known state specific multireference coupled cluster (SS-MRCC) method introduced by Mahapatra et al. [Mol. Phys. 94, 157 (1998)]. As that of the SS-MRCC theory, the present method is also based on the Jeziorski-Monkhorst ansatz where a different exponential cluster operator exp(T(mu)) acts on its corresponding model function phi(mu). The final cluster finding equations contain coupling between the cluster operators for all the mu, which are mainly responsible to prove the extensivity of both the cluster amplitudes and the energy. The present sr-MRCC theory is size-extensive and size-consistent when localized orbitals are used. The systems considered here exhibit varying degrees of degeneracy at different regions of PES. The treatment of these systems via traditional effective Hamiltonian based methods suffers from divergence problems in the iterative solution of the CC equations (the issue termed as "intruder state"). The sr-MRCC results lie closer to the ones obtained by the SS-MRCC method for these systems. To judge the efficacy of the present method, we have compared our results with other previously published theoretical estimations, which clearly indicate that the present method is reliable in studying the dissociation PES of states plagued by electronic degeneracy as well as notorious intruder effects. The highly satisfactory performance of the sr-MRCC method, vis-a-vis the other sophisticated methods, in describing the lowest and the first excited singlet states of BeH(2) at points of high degeneracy is noticeable.
我们采用完全活性空间基于单根多参考耦合簇方法（简称 sr-MRCC）来计算一些经过深入研究的“典型模型”系统的势能面（PES），这些系统有高度准确和可靠的数据库可供比较。与状态特定理论一样，sr-MRCC 方法专注于关联一个状态，同时使用多组态参考，因此它自然避免了侵入态。本方法在结构上不同于 Mahapatra 等人引入的著名的状态特定多参考耦合簇（SS-MRCC）方法[Mol. Phys. 94, 157 (1998)]。与 SS-MRCC 理论一样，本方法也是基于 Jeziorski-Monkhorst 假设，其中不同的指数簇算符 exp(T(mu))作用于其对应的模型函数 phi(mu)。最终的簇发现方程包含了所有 mu 的簇算符之间的耦合，这些耦合主要负责证明簇振幅和能量的扩展性。当使用局域轨道时，本 sr-MRCC 理论是大小扩展性和大小一致性的。这里考虑的系统在 PES 的不同区域表现出不同程度的简并。通过传统的基于有效哈密顿量的方法处理这些系统，在 CC 方程的迭代求解中会出现发散问题（称为“侵入态”）。sr-MRCC 的结果更接近 SS-MRCC 方法对这些系统的结果。为了判断本方法的有效性，我们将我们的结果与其他先前发表的理论估计进行了比较，这清楚地表明，本方法在研究受电子简并以及臭名昭著的侵入态效应困扰的状态的离解 PES 方面是可靠的。在描述 BeH(2)的最低和第一激发单重态在高度简并点时，sr-MRCC 方法与其他复杂方法相比表现出非常令人满意的性能。
