MTA-BME Lendület Quantum Chemistry Research Group, Department of Physical Chemistry and Materials Science , Budapest University of Technology and Economics , H-1521 Budapest , Hungary.
Department of Inorganic and Analytical Chemistry , Budapest University of Technology and Economics , H-1521 Budapest , Hungary.
J Chem Theory Comput. 2018 May 8;14(5):2469-2479. doi: 10.1021/acs.jctc.8b00072. Epub 2018 Apr 5.
We analyzed various possibilities to improve upon the SCAN meta-generalized gradient approximation density functional obeying all known properties of the exact functional that can be satisfied at this level of approximation. We examined the necessity of locally satisfying a strongly tightened lower bound for the exchange energy density in single-orbital regions, the nature of the error cancellation between the exchange and correlation parts in two-electron regions, and the effect of the fourth-order term in the gradient expansion of the correlation energy density. We have concluded that the functional can be modified to separately reproduce the exchange and correlation energies of the helium atom by locally releasing the strongly tightened lower bound for the exchange energy density in single-orbital regions, but this leads to an unbalanced improvement in the single-orbital electron densities. Therefore, we decided to keep the F ≤ 1.174 exact condition for any single-orbital density, where F is the exchange enhancement factor. However, we observed a general improvement in the single-orbital electron densities by revising the correlation functional form to follow the second-order gradient expansion in a wider range. Our new revSCAN functional provides more-accurate atomization energies for the systems with multireference character, compared to the SCAN functional. The nonlocal VV10 dispersion-corrected revSCAN functional yields more-accurate noncovalent interaction energies than the VV10-corrected SCAN functional. Furthermore, its global hybrid version with 25% of exact exchange, called revSCAN0, generally performs better than the similar SCAN0 for reaction barrier heights. Here, we also analyzed the possibility of the construction of a local hybrid from the SCAN exchange and a specific locally bounded nonconventional exact exchange energy density. We predict compatibility problems since this nonconventional exact exchange energy density does not really obey the strongly tightened lower bound for the exchange energy density in single-orbital regions.
我们分析了各种可能的方法来改进 SCAN 元广义梯度近似密度泛函，使其符合在这种近似水平下可以满足的精确泛函的所有已知性质。我们研究了在单轨道区域局部满足交换能密度强紧下限的必要性、双电子区域中交换和相关部分之间误差消除的性质，以及相关能量密度梯度展开中四阶项的影响。我们得出的结论是，可以通过在单轨道区域局部释放交换能密度的强紧下限来修改泛函，以分别再现氦原子的交换和相关能，但这会导致单轨道电子密度的不平衡改善。因此，我们决定保持任何单轨道密度的 F ≤ 1.174 的精确条件，其中 F 是交换增强因子。然而，通过在更广泛的范围内修改相关泛函形式以遵循二阶梯度展开，我们观察到单轨道电子密度的普遍改善。与 SCAN 泛函相比，新的 revSCAN 泛函为具有多参考特征的体系提供了更准确的原子化能。非局部 VV10 色散校正的 revSCAN 泛函产生的非共价相互作用能比 VV10 校正的 SCAN 泛函更准确。此外，其具有 25%精确交换的全局混合版本，称为 revSCAN0，通常比类似的 SCAN0 更适合反应势垒高度。在这里，我们还分析了从 SCAN 交换和特定局部有界非常规精确交换能密度构建局部混合的可能性。我们预测会出现兼容性问题，因为这种非常规精确交换能密度实际上并不遵守单轨道区域交换能密度的强紧下限。
