XTB 的 ONIOM 方法

There is no reason to be constrained to XTB as the low-level method during a ONIOM calculation. Combinations of any two methods available in ORCA can be chosen! Let us explore some of the possibilities studying the transition state energy of the follow dehydration reaction:
在 ONIOM 计算过程中，没有理由局限于将 XTB 作为低级方法。可以选择 ORCA 中任意两种方法的组合！让我们通过研究以下脱水反应的过渡态能量来探讨一些可能性：

The reactant and transition state were taken from the BHDIV10 set, a subset of the GMTKN55 benchmark set [Grimme2017b]. Geometries for the educt (ed1) and transition state (ts1) are from here.
反应物和过渡态取自 BHDIV10 集合，这是 GMTKN55 基准集合的一个子集[Grimme2017b]。反应物（ed1）和过渡态（ts1）的几何结构均来源于此。

Example 1: Mix of hybrid and non-hybrid DFT#
示例 1：混合型与非混合型 DFT 的组合

Let's first try a combination of two DFT methods: a high-level hybrid wB97X-D3/DEF2-TZVP and a lower-level PBE/DEF2-SVP. Our split of the QM regions will be again considering where the reaction does take place and which parts needs to be modeled with more accuracy.
首先尝试两种 DFT 方法的组合：高层次的混合方法 wB97X-D3/DEF2-TZVP 和较低层次的 PBE/DEF2-SVP。我们将再次根据反应实际发生的位置以及需要更精确建模的部分来划分 QM 区域。

In this particular case, the dehydration reaction is rather localized, and we will split our system as:
在此特定情况下，脱水反应较为局部化，我们将系统划分为：

which, as an ORCA input is simply written as:
作为 ORCA 输入，其简单写法为：

!QM/QM2 WB97X-D3 DEF2-TZVP
%QMMM QM2CUSTOMMETHOD "PBE D3BJ DEF2-SVP"
      QMATOMS {2:3} {6:13} END
END
* XYZ 0 1
(...)

The QM/QM2 flag indicates that we will again use two different QM methods. The first, high-level one is simply defined in the same input line as always, here wB97X-D3 DEF2-TZVP. The second is given under %QMMM after QM2CUSTOMMETHOD, as a string, which here was chosen as PBE D3BJ DEF2-SVP.
QM/QM2 标志表示我们将再次使用两种不同的 QM 方法。第一种高级别方法在输入行中一如既往地定义，此处为 wB97X-D3 DEF2-TZVP。第二种方法在%QMMM 下通过 QM2CUSTOMMETHOD 指定，以字符串形式给出，此处选择为 PBE D3BJ DEF2-SVP 。

The QMATOMS list is then used to define the high-level region, and all the others are assigned to the lower-level region. The results here is then $E_{b a r r i e r}^{O N I O M} = 24.29$ $k c a l / m o l$ , which is very close to the full wB97X-D3/DEF2-TZVP result of $E_{b a r r i e r}^{w B 97 X - D 3} = 24.34$ $k c a l / m o l$ .
然后使用 QMATOMS 列表定义高级别区域，其余部分则分配给低级别区域。此处结果为 $E_{b a r r i e r}^{O N I O M} = 24.29$ $k c a l / m o l$ ，与全 wB97X-D3/DEF2-TZVP 结果 $E_{b a r r i e r}^{w B 97 X - D 3} = 24.34$ $k c a l / m o l$ 非常接近。

Methods to compute the charges of the QM2 region#
计算 QM2 区域电荷的方法

Looking closely at the ONIOM header printed in the very beginning, you might have noticed the reference to Hirshfeld charges:
仔细观察最初打印的 ONIOM 头文件，您可能已经注意到对 Hirshfeld 电荷的引用：

QM2 method                             ... Custom-QM2
        QM2 method                     ... PBE D3BJ DEF2-SVP
        QM2 basis                      ...
Coupling Scheme                        ... subtractive
Embedding Scheme                       ... electrostatic
PrintLevel                             ... 1
Method for determining QM2 charges     ... Hirshfeld
Charge of total system                 ... 0

This is related to how the charges are computed at the lower level (QM2), which will later then be used to compute the higher-level method (QM1), as necessary for the electrostatic embedding (What is "electrostatic embedding"?).
这与较低层次（QM2）的电荷计算方式有关，这些计算结果随后将用于必要时的高层次方法（QM1）计算，以实现静电嵌入（什么是“静电嵌入”？）。

In case needed, these can be changed by setting Charge_Method under %QMMM, and the options are:
如有需要，可通过设置%QMMM 下的 Charge_Method 进行更改，选项包括：

%QMMM Charge_Method     Hirshfeld # (default)
                        # CHELPG
                        # Mulliken
                        # Loewdin (default for QM2 = AM1 or PM3)
END

For XTB, the charges used are the ones calculated directly via the XTB method. We don't recommend changing these, but the options are laid out here for completion. More details can be found, as always, in the ORCA manual.
对于 XTB，所使用的费用是直接通过 XTB 方法计算得出的。我们不建议更改这些费用，但为了完整性，这里列出了相关选项。如往常一样，更多详细信息可以在 ORCA 手册中找到。

Example 2: Mix of CCSD(T) and DFT#
示例 2：CCSD(T) 与 DFT 混合方法

The results obtained with mixed DFT methods ( $E_{b a r r i e r}^{O N I O M} = 24.29$ $k c a l / m o l$ ) were not bad compared to the reference values obtained from extrapolated coupled-cluster with a large basis $E_{b a r r i e r}^{R e f} = 25.65$ $k c a l / m o l$ .
采用混合密度泛函方法（ $E_{b a r r i e r}^{O N I O M} = 24.29$ $k c a l / m o l$ ）得到的结果与通过大基组外推耦合簇方法获得的参考值相比并不逊色。

However, $1.4$ $k c a l / m o l$ in an energy barrier is an error of one order of magnitude on the resulting rate constant, and maybe that could be improved.
然而，能量势垒中一个数量级的误差会导致速率常数结果相差一个数量级，或许这可以得到改进。

We can actually do that by combining CCSD(T) and DFT in a single run! In particular, we can combine here DLPNO-CCSD(T) and DFT using:
我们实际上可以通过将 CCSD(T)和 DFT 结合在一次运行中实现这一点！特别是，我们可以在此结合 DLPNO-CCSD(T)和 DFT 使用：

!QM/QM2 DLPNO-CCSD(T) DEF2-TZVP DEF2-TZVP/C RIJCOSX
%QMMM QM2CUSTOMMETHOD "PBE D3BJ DEF2-SVP"
      QMATOMS {2:3} {6:13} END
END
* XYZ 0 1
(...)

More details related to the coupled-cluster can be found in the Correlation energy section, but it is important to say that, using !PAL16 to use 16 cores these calculations took only 30 seconds!
有关耦合簇的更多细节可在相关能部分找到，但值得一提的是，使用!PAL16 调用 16 个核心进行这些计算仅需 30 秒！

The energy obtained from the CCSD(T)/DFT ONIOM is found to be $E_{b a r r i e r}^{O N I O M} = 25.48$ $k c a l / m o l$ , which is very close to the reference value, wrong by only $0.17$ $k c a l / m o l$ . Here is a table with values from a few combinations:
从 CCSD(T)/DFT ONIOM 获得的能量为 $E_{b a r r i e r}^{O N I O M} = 25.48$ $k c a l / m o l$ ，与参考值非常接近，仅相差 $0.17$ $k c a l / m o l$ 。以下是几种组合的数值表：

Comparison of different methods to compute the transition state energy for the first system of the BHDIV10 set.
BHDIV10 集合中第一个系统的过渡态能量计算方法比较。#
Method 方法	$E_{b i n d i n g}$ kcal/mol $E_{b i n d i n g}$ 千卡/摩尔
full wB97X-D3 全 wB97X-D3	24.34
full XTB 全 XTB	32.73
wB97X-D3/XTB	24.06
wB97X-D3/PBE	24.49
wB97X-D3/wB97X-D3	24.34
DLPNO-CCSD(T)/PBE	25.48
DLPNO-CCSD(T)/wB97X-D3	25.34
Reference [Grimme2017b] 参考文献 [Grimme2017b]	25.65

Link atoms# 链接原子

You might be wondering how does ORCA treat the broken bonds when doing a multiscale calculation. After all, we have split the molecule here in two, and treated each part on a different level.
您可能会好奇，在进行多尺度计算时，ORCA 如何处理断裂的键。毕竟，我们在这里将分子一分为二，并对每个部分采用了不同的处理层次。

In QMMM or ONIOM approaches, this is solved using the idea of "link atoms" [Hobza2011]. The approach is to substitute single bonds between any two atoms by a atom-hydrogen bond on both sides. There are some predefined rules for how to do it, and the number of link atoms is printed in the beginning of the output, on the ONIOM header:
在 QMMM 或 ONIOM 方法中，这一问题通过使用“链接原子”的概念得以解决[Hobza2011]。具体做法是将任意两个原子间的单键替换为两侧的氢原子键。存在一些预定义的规则来指导如何操作，并且在输出开始部分的 ONIOM 标题中会打印出链接原子的数量。

Size of different subsystems (in atoms):
Size of QMMM System                    ... 16
Size of QM2 Subsystem                  ... 6
Size of QM1 Subsystem                  ... 10
Number of link atoms                   ... 1
Size of QM1 Subsystem plus link atoms  ... 11
Size of region for optimizer           ... 16
        optimized atoms = activeRegion ... 16

And here we have only 1 link atom. Looking closely at the higher level QM calculation output, the molecule that is actually calculated on that level is:
这里我们只有一个连接原子。仔细观察更高层次的量子力学计算输出，实际在该层次上计算的分子是：

There is nothing to worry about this and ORCA does all the necessary corrections internally, but in case you need more details or want to change something, please check the ORCA manual.
对此无需担忧，ORCA 会自动进行所有必要的内部修正，但若您需要更多详情或希望调整某些设置，请查阅 ORCA 手册。

Important 重要

When defining the QM1 and QM2 regions, you can not: 在定义 QM1 和 QM2 区域时，您不能：

cut through a bond with a hydrogen, for it would be substituted by a H link atom anyway.
通过氢切断一个键，因为它无论如何都会被一个 H 连接原子所取代。
cut through a double/triple/multiple bond, for the link atom approach would not be able to fix that.
切断双键/三键/多重键，因为链接原子方法无法解决这一问题。

ONIOM methods beyond QM/XTB#超越 QM/XTB 的 ONIOM 方法

Example 1: Mix of hybrid and non-hybrid DFT#示例 1：混合型与非混合型 DFT 的组合

Methods to compute the charges of the QM2 region#计算 QM2 区域电荷的方法

Example 2: Mix of CCSD(T) and DFT#示例 2：CCSD(T) 与 DFT 混合方法

Link atoms# 链接原子

Structures# 结构

ONIOM methods beyond QM/XTB#
超越 QM/XTB 的 ONIOM 方法

Example 1: Mix of hybrid and non-hybrid DFT#
示例 1：混合型与非混合型 DFT 的组合

Methods to compute the charges of the QM2 region#
计算 QM2 区域电荷的方法

Example 2: Mix of CCSD(T) and DFT#
示例 2：CCSD(T) 与 DFT 混合方法