Evangelista Francesco A, Allen Wesley D, Schaefer Henry F
Center for Computational Chemistry, University of Georgia, Athens, Georgia 30602-2556, USA.
J Chem Phys. 2006 Oct 21;125(15):154113. doi: 10.1063/1.2357923.
For the first time high-order excitations (n>2) have been studied in three multireference couple cluster (MRCC) theories built on the wave operator formalism: (1) the state-universal (SU) method of Jeziorski and Monkhorst (JM) (2) the state-specific Brillouin-Wigner (BW) coupled cluster method, and (3) the state-specific MRCC approach of Mukherjee (Mk). For the H4, P4, BeH(2), and H8 models, multireference coupled cluster wave functions, with complete excitations ranging from doubles to hextuples, have been computed with a new arbitrary-order string-based code. Comparison is then made to corresponding single-reference coupled cluster and full configuration interaction (FCI) results. For the ground states the BW and Mk methods are found, in general, to provide more accurate results than the SU approach at all levels of truncation of the cluster operator. The inclusion of connected triple excitations reduces the nonparallelism error in singles and doubles MRCC energies by a factor of 2-10. In the BeH(2) and H8 models, the inclusion of all quadruple excitations yields absolute energies within 1 kcal mol(-1) of the FCI limit. While the MRCC methods are very effective in multireference regions of the potential energy surfaces, they are outperformed by single-reference CC when one electronic configuration dominates.
首次在基于波动算符形式主义构建的三种多参考耦合簇（MRCC）理论中研究了高阶激发（n>2）：（1）Jeziorski和Monkhorst（JM）的状态通用（SU）方法；（2）状态特定的布里渊 - 维格纳（BW）耦合簇方法；（3）Mukherjee（Mk）的状态特定MRCC方法。对于H4、P4、BeH(2)和H8模型，使用一种新的基于任意阶串的代码计算了从双激发到六重激发的完整多参考耦合簇波函数。然后将其与相应的单参考耦合簇和全组态相互作用（FCI）结果进行比较。对于基态，一般发现BW和Mk方法在簇算符的所有截断水平上都比SU方法提供更准确的结果。包含连接的三重激发将单双激发MRCC能量中的非平行性误差降低了2到10倍。在BeH(2)和H8模型中，包含所有四重激发产生的绝对能量在FCI极限的1 kcal mol(-1)范围内。虽然MRCC方法在势能面的多参考区域非常有效，但当一种电子组态占主导时，它们的表现不如单参考CC方法。
