从Gview 操作来看，

ONIOM 分为2场的时候

这个是对的。

电荷和多重度 对应的是 整个体系/高场体系/高场体系

从手册和 https://gaussian.com/oniom/   操作来看，

ONIOM 分为2场的时候

电荷和多重度 对应的是 整个体系/高场体系/model-low场体系

往往会错误的想当然的认为model-low 就是低场。

ONIOM中没有对低场电荷和多重度的描述

model层一般和high层一般是一致的。

The High Layer is the smallest one, and it is treated with the most accurate method. Bond formation and breaking takes place in this region. This layer is also called the Small Model System (SM). In a 2-layer ONIOM calculation, it is often simply called the Model System.

High layer == Small Model == Model System

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三层很难收敛，而且耗时很难估计。 建议使用两层进行研究体系。

因为跑一圈H层，L层要跑好多圈都不一定收敛，这样跑N圈下来计算量不是一般的大了。

Per-Layer Charges and Spin Multiplicities

Multiple charge and spin multiplicity pairs may also be specified for ONIOM calculations. For two-layer ONIOM jobs, the format for this input line is:

charge^low(R) spin^low(R)   charge^high(SM) spin^high(SM)   charge^low(SM) spin^low(SM)   [ charge^high(R) spin^high(R) ]

这里low high 和高场 低场没有关系，区分理论计算方法MM 和QM。

SM代表的是高场，R代表的是整个体系。

第一对是整个体系的电荷和多重度，用在MM理论 进行计算；

第二对是高层的电荷和多重度，用QM理论进行计算；

第三对是高层的电荷和多重度，用MM理论进行计算;

第四对是真实体系的电荷和多重度， 仅适用于ONIOM = SValue计算，才需要第四对。

The first pair of numbers, –11 and 1, are the charge and spin multiplicity for the low level of theory (MM in this case) calculation of the real system (including both QM and MM regions in this case). The second pair of numbers, 1 and 1, are the charge and spin multiplicity for high level of theory (QM) calculation of the model system (QM region in this case). The third pair of numbers, 1 and 1, are the charge and spin multiplicity for the low level of theory (MM) calculation of the model system (QM region).

where the superscript indicates the model chemistry and the parameter the layer for which the value will be used. The fourth pair applies only to ONIOM=SValue calculations. When only a single value pair is specified, all levels will use those values. If two pairs of values are included, then the third pair defaults to the same values as in the second pair. If the final pair is omitted for an S-Value job, it defaults to the values for the real system at the low level.

Values and defaults for three-layer ONIOM calculations follow an analogous pattern. See the ONIOM keyword in the Gaussian 16 User’s Reference for full details.

总结

ONIOM 分为2场的时候

这个是对的。

电荷和多重度 对应的是 整个体系/高场体系/高场体系

参考

1.   http://gaussian.com/oniom_technote/

2.