Polarization propagators can be identified with linear responses of a system to external/internal perturbing fields, being that responses experimentally measurable. Then, approximated and reliable calculations with propagators requires to fulfill some well defined constraints like the invariance under unitary transformations, UT. Polarization propagators are not invariant under UT of molecular orbitals at second-order, known as SOPPA. We found the origin of such non invariance and propose a modification to partially cure this deficiency. Calculations with the original SOPPA model and the new one of NMR shieldings in small saturated and unsaturated systems show that differences appears for the unsaturated ones.

极化传播器可以通过系统对外部/内部扰动场的线性响应来识别，因为该响应可以通过实验测量。然后，使用传播子进行近似和可靠的计算需要满足一些明确定义的约束，例如unit变换下的不变性UT。偏振传播器在分子轨道的UT下处于二阶（称为SOPPA）不变。我们找到了这种不变性的根源，并提出了一种修改方案以部分治愈这种缺陷。用原始的SOPPA模型和新的NMR屏蔽在小的饱和和不饱和体系中进行的计算表明，不饱和体系存在差异。