Ramos-Cordoba Eloy, Lopez Xabier, Piris Mario, Matito Eduard
Faculty of Chemistry, University of the Basque Country UPV/EHU, and Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi, Spain.
J Chem Phys. 2015 Oct 28;143(16):164112. doi: 10.1063/1.4934799.
The correct description of nondynamic correlation by electronic structure methods not belonging to the multireference family is a challenging issue. The transition of D(2h) to D(4h) symmetry in H4 molecule is among the most simple archetypal examples to illustrate the consequences of missing nondynamic correlation effects. The resurgence of interest in density matrix functional methods has brought several new methods including the family of Piris Natural Orbital Functionals (PNOF). In this work, we compare PNOF5 and PNOF6, which include nondynamic electron correlation effects to some extent, with other standard ab initio methods in the H4 D(4h)/D(2h) potential energy surface (PES). Thus far, the wrongful behavior of single-reference methods at the D(2h)-D(4h) transition of H4 has been attributed to wrong account of nondynamic correlation effects, whereas in geminal-based approaches, it has been assigned to a wrong coupling of spins and the localized nature of the orbitals. We will show that actually interpair nondynamic correlation is the key to a cusp-free qualitatively correct description of H4 PES. By introducing interpair nondynamic correlation, PNOF6 is shown to avoid cusps and provide the correct smooth PES features at distances close to the equilibrium, total and local spin properties along with the correct electron delocalization, as reflected by natural orbitals and multicenter delocalization indices.
用不属于多参考系族的电子结构方法正确描述非动态相关是一个具有挑战性的问题。H4分子中从D(2h)到D(4h)对称性的转变是说明缺失非动态相关效应后果的最简单典型例子之一。对密度矩阵泛函方法兴趣的复苏带来了几种新方法，包括皮里斯自然轨道泛函（PNOF）族。在这项工作中，我们将在H4的D(4h)/D(2h)势能面（PES）中，将在一定程度上包含非动态电子相关效应的PNOF5和PNOF6与其他标准从头算方法进行比较。到目前为止，单参考系方法在H4的D(2h)-D(4h)转变处的错误行为一直归因于对非动态相关效应的错误处理，而在基于双电子对的方法中，它被归因于自旋的错误耦合和轨道的定域性质。我们将表明，实际上双电子对间非动态相关是对H4势能面进行无尖点定性正确描述的关键。通过引入双电子对间非动态相关，PNOF6被证明可以避免尖点，并在接近平衡的距离处提供正确的平滑势能面特征、总自旋和局部自旋性质以及正确的电子离域，这由自然轨道和多中心离域指数反映出来。
