Department of Chemistry, University of California, Berkeley, California 94720, USA.
J Chem Phys. 2013 Dec 28;139(24):244109. doi: 10.1063/1.4851816.
Orbital-optimized second-order perturbation theory (OOMP2) optimizes the zeroth order wave function in the presence of correlations, removing the dependence of the method on Hartree-Fock orbitals. This is particularly important for systems where mean field orbitals spin contaminate to artificially lower the zeroth order energy such as open shell molecules, highly conjugated systems, and organometallic compounds. Unfortunately, the promise of OOMP2 is hampered by the possibility of solutions being drawn into divergences, which can occur during the optimization procedure if HOMO and LUMO energies approach degeneracy. In this work, we regularize these divergences through the simple addition of a level shift parameter to the denominator of the MP2 amplitudes. We find that a large level shift parameter of 400 mEh removes divergent behavior while also improving the overall accuracy of the method for atomization energies, barrier heights, intermolecular interactions, radical stabilization energies, and metal binding energies.
轨道优化的二级微扰理论（OOMP2）在相关性存在的情况下优化了零级波函数，消除了该方法对哈特利-福克轨道的依赖性。对于那些平均场轨道自旋污染导致零级能量人为降低的体系，如开壳分子、高度共轭体系和有机金属化合物，这一点尤为重要。不幸的是，OOMP2 的前景受到了可能出现的解决方案被吸引到发散的阻碍，这种情况可能会在优化过程中发生，如果 HOMO 和 LUMO 能量接近简并。在这项工作中，我们通过在 MP2 振幅的分母中简单地添加一个能级移动参数来正则化这些发散。我们发现，一个较大的能级移动参数 400 mEh 可以消除发散行为，同时也提高了该方法在原子化能、势垒高度、分子间相互作用、自由基稳定能和金属结合能方面的整体准确性。
