Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany.
J Chem Phys. 2010 Nov 7;133(17):174113. doi: 10.1063/1.3503041.
A rigorous perturbation theory is proposed, which has the same second order energy as the spin-component-scaled Møller-Plesset second order (SCS-MP2) method of Grimme [J. Chem. Phys. 118, 9095 (2003)]. This upgrades SCS-MP2 to a systematically improvable, true wave-function-based method. The perturbation theory is defined by an unperturbed Hamiltonian, Ĥ(0), that contains the ordinary Fock operator and spin operators Ŝ(2) that act either on the occupied or the virtual orbital spaces. Two choices for Ĥ(0) are discussed and the importance of a spin-pure Ĥ((0)) is underlined. Like the SCS-MP2 approach, the theory contains two parameters (c(os) and c(ss)) that scale the opposite-spin and the same-spin contributions to the second order perturbation energy. It is shown that these parameters can be determined from theoretical considerations by a Feenberg scaling approach or a fit of the wave functions from the perturbation theory to the exact one from a full configuration interaction calculation. The parameters c(os)=1.15 and c(ss)=0.75 are found to be optimal for a reasonable test set of molecules. The meaning of these parameters and the consequences following from a well defined improved MP method are discussed.
提出了一个严格的微扰理论，它具有与 Grimme 的自旋分量标度 Møller-Plesset 二阶（SCS-MP2）方法相同的二阶能量[J. Chem. Phys. 118, 9095 (2003)]。这将 SCS-MP2 升级为一种系统可改进的、真正基于波函数的方法。微扰理论由未微扰哈密顿量Ĥ(0)定义，该哈密顿量包含通常的福克算子和自旋算子Ŝ(2)，它们作用于占据轨道空间或虚拟轨道空间。讨论了两种Ĥ(0)的选择，并强调了自旋纯Ĥ((0))的重要性。与 SCS-MP2 方法一样，该理论包含两个参数（c(os)和c(ss)），这些参数对二阶微扰能的相反自旋和相同自旋贡献进行缩放。结果表明，这些参数可以通过芬恩伯格缩放方法或通过将微扰理论中的波函数拟合到全组态相互作用计算的精确波函数来从理论上确定。发现参数 c(os)=1.15 和 c(ss)=0.75 对于合理的分子测试集是最佳的。讨论了这些参数的含义以及从定义良好的改进 MP 方法得出的结果。
