Department of Physics, Taki Government College, Taki, North 24 Parganas 743429, India.
J Chem Phys. 2011 Jan 28;134(4):044113. doi: 10.1063/1.3523573.
The complete model space (CAS) based "genuine" single root multireference (MR) coupled cluster (sr-MRCC) method [Mahapatra and Chattopadhyay, J. Chem. Phys. 133, 074102 (2010)] has been extended to enable geometry optimizations by adopting the numerical gradient scheme. The sr-MRCC theory is designed to treat quasidegeneracies of varying degrees through the computation of essential static and dynamic correlation effects in a balanced way while bypassing the intruder states problem in a size-extensive manner. The efficacy of our sr-MRCC gradient approach has been illustrated by the optimization of the geometries of N(2)H(2),CH(2),C(2)H(4),C(4)H(4),O(3) as well as trimethylenemethane (TMM) molecular systems, since such cases, by virtue of their complexity, warrant truly multireference description. We have explored the capability of the sr-MRCC approach to yield rotational energy surfaces for the ground and first singlet excited states of N(2)H(2). We also intend to explore the ground and the excited state energetics of some model systems (such as P4, H4, and H(8)) for the computation of excitation energies by relying on the sr-MRCC method. An analysis of the results and a comparison with previous pertinent theoretical works including state specific MRCC (SS-MRCC) theory of Mukherjee and co-workers have also been presented. Although in most of the cases, we observe a close behavior between the sr-MRCC and SS-MRCC method, the error in the sr-MRCC is lower than the overall error of the SS-MRCC calculations in the vicinity of the transition region (manifesting a significant quasidegenerate character). The present results show that the sr-MRCC method and its numerical gradient variant are generally applicable to very demanding model and realistic chemical problems at acceptable accuracy and affordable computational expense which together attests the efficacy and viability of the sr-MRCC formalism for handling of static and dynamic correlations simultaneously thereby ensuring a balanced description for bond-breaking and other quasidegenerate situations with a various degree of MR character. Our preliminary results illustrate that our sr-MRCC method is a potential competitor for other state specific MRCC theories.
基于完全模型空间（CAS）的“真实”单根多参考（sr-MRCC）耦合簇方法[Mahapatra 和 Chattopadhyay，J. Chem. Phys. 133, 074102（2010）]已被扩展，通过采用数值梯度方案实现几何优化。sr-MRCC 理论旨在通过平衡计算基本静态和动态相关效应来处理不同程度的准简并性，同时以尺寸扩展性方式绕过侵入态问题。我们的 sr-MRCC 梯度方法的有效性已通过优化 N(2)H(2)、CH(2)、C(2)H(4)、C(4)H(4)、O(3)以及三甲烯甲烷（TMM）分子系统的几何形状得到证明，因为这些情况由于其复杂性，需要真正的多参考描述。我们已经探索了 sr-MRCC 方法为 N(2)H(2)的基态和第一单重激发态生成旋转能面的能力。我们还打算探索一些模型系统（如 P4、H4 和 H(8)）的基态和激发态能，以便依靠 sr-MRCC 方法计算激发能。还对结果进行了分析，并与 Mukherjee 及其同事的状态特定 MRCC（SS-MRCC）理论等以前的相关理论工作进行了比较。尽管在大多数情况下，我们观察到 sr-MRCC 和 SS-MRCC 方法之间的密切行为，但在过渡区域附近，sr-MRCC 的误差低于 SS-MRCC 计算的总误差（表现出明显的准简并特征）。这些结果表明，sr-MRCC 方法及其数值梯度变体通常适用于非常苛刻的模型和实际化学问题，在可接受的精度和可负担的计算费用下，这共同证明了 sr-MRCC 形式在处理静态和动态相关方面的有效性和可行性，从而确保了对键断裂和其他具有不同程度 MR 特征的准简并情况的平衡描述。我们的初步结果表明，我们的 sr-MRCC 方法是其他状态特定 MRCC 理论的潜在竞争对手。
