Intrinsic Reaction Coordinate (IRC)#
内禀反应坐标 (IRC)

The Intrinsic Reaction Coordinate (IRC) is a special form of a minimum energy path that connects a transition state (TS) with its downhill-nearest intermediates. A method determining the IRC is thus useful to determine whether a transition state is directly connected to a given reactant and/or product. ORCA features its own implementation of Morokuma and coworkers’ popular method.[Morokuma1977] The IRC method can be requested via the IRC simple keyword:
内禀反应坐标（IRC）是一种特殊形式的最小能量路径，它连接过渡态（TS）与其下坡最近的中间体。因此，确定 IRC 的方法有助于判断一个过渡态是否直接连接到给定的反应物和/或产物。ORCA 实现了 Morokuma 及其同事的流行方法。[Morokuma1977] 可以通过 IRC 简单关键词请求 IRC 方法：

!r2SCAN-3c IRC

*XYZFILE 0 1 ts.xyz

Example: C-H Activation#
示例：C-H 键活化

In this example, we will compute the IRC for a transition state of the oxidative addition of methane to a simplified version of Vaska's complex trans-IrCl(CO)[P(CH₃)₃]₂. The TS was previously found and refined with the NEB-TS method.
在此示例中，我们将计算甲烷氧化加成至简化版 Vaska 配合物 trans-IrCl(CO)[P(CH ₃ ) ₃ ] ₂ 的过渡态的 IRC。该过渡态先前已通过 NEB-TS 方法找到并优化。

Note 注释

Note that the IRC method can be directly requested together with the OPTTS, SCANTS, NEB-TS, FREQ and NUMFREQ keywords. Per default the IRC code checks whether a Hessian was computed before the IRC run. If that is not the case, and if no Hessian is provided via the %IRC block, a new Hessian is computed at the beginning of the IRC run.
请注意，IRC 方法可与 OPTTS 、 SCANTS 、 NEB-TS 、 FREQ 和 NUMFREQ 关键字直接一同请求。默认情况下，IRC 代码会检查在 IRC 运行之前是否已计算 Hessian 矩阵。若未计算且未通过 %IRC 块提供 Hessian 矩阵，则在 IRC 运行开始时会重新计算 Hessian 矩阵。

Important 重要

The same method and basis set as used for optimization and frequency calculation should be used for the IRC run!
应使用与优化和频率计算相同的基组和方法进行 IRC 计算！

We now request the IRC calculation on our given transition state. Note that we exemplarily increased the number of maximum iterations from 20 (default) to 30 via the %IRC block. More details on available IRC settings can be found in the respective section of the ORCA manual.
我们现在请求对给定的过渡态进行 IRC 计算。请注意，我们通过 %IRC 块将最大迭代次数从默认的 20 次增加到了 30 次作为示例。有关可用 IRC 设置的更多详细信息，请参阅 ORCA 手册中的相应部分。

!r2SCAN-3c IRC

%IRC
 MAXITER 30
END

*XYZFILE 0 1 ts.xyz

After an initial energy and gradient calculation, the IRC procedure is initiated:
在初始能量和梯度计算之后，IRC 程序启动：

--------------------------------------------------------------------------------
                   Intrinsic Reaction Coordinate Calculation
--------------------------------------------------------------------------------

System:
Nr. of atoms                        .... 17
Algorithm: SD (steepest descent step) plus correction
Settings:
Max. no of cycles        MaxIter    .... 50
Direction                           .... Forward and backward
Initial displacement Hessian type   .... Compute numerically
Following Hessian mode              .... 0
Initial displacement type           .... Energy
  Initial displacement energy change.... 2.000 mEh
Coordinate system following type    .... Cartesian
Length of steepest descent step     .... 0.150 bohr
Do parabolic fit
  if SD step is uphill              .... YES
Do Correction to SD step            .... YES
Scaling for length of correction
  compared to SD step               .... 0.333
Do parabolic fit to SD correction   .... YES
Only interpolate for parabolic fit  .... YES
Do update to length of
  SD step and correction            .... YES
Convergence Tolerances:
  Max. Gradient            TolMAXG  ....  2.0000e-03 Eh/bohr
  RMS Gradient             TolRMSG  ....  5.0000e-04 Eh/bohr
Storing full IRC trajectory in      .... orca_IRC_Full_trj.xyz
Storing forward trajectory in       .... orca_IRC_F_trj.xyz
Storing backward trajectory in      .... orca_IRC_B_trj.xyz

As we did not provide any Hessian or specify the Hessian calculation in our input, ORCA will perform a numerical Hessian calculation first:
由于我们在输入中未提供任何 Hessian 矩阵或指定其计算方法，ORCA 将首先执行数值 Hessian 计算：

----------------------------------------------------------------------------
                           ORCA NUMERICAL FREQUENCIES
----------------------------------------------------------------------------

Afterwards, the computed Hessian will be read automatically and the forward IRC calculation will start:
随后，计算得到的 Hessian 矩阵将被自动读取，并开始正向 IRC 计算：

         *************************************************************
         *                          FORWARD IRC                      *
         *************************************************************

Iteration    E(Eh)      dE(kcal/mol)  max(|G|)   RMS(G)
Convergence thresholds                0.002000  0.000500
    0     -1404.718702   -0.809101    0.020193  0.004018
[...]

Next, the backward IRC is computed:
接下来，计算反向 IRC：

         *************************************************************
         *                          BACKWARD IRC                     *
         *************************************************************

Iteration    E(Eh)      dE(kcal/mol)  max(|G|)   RMS(G)
Convergence thresholds                0.002000  0.000500
    0     -1404.719178   -1.108245    0.014077  0.003383
[...]

Finally, the IRC path is summarized:
最后，IRC 路径总结如下：

---------------------------------------------------------------
                       IRC PATH SUMMARY
---------------------------------------------------------------
All gradients are in Eh/Bohr.

Step        E(Eh)      dE(kcal/mol)  max(|G|)   RMS(G)
   1     -1404.757622   -25.231708    0.004829  0.001679
[...]
  31     -1404.717412    0.000000    0.000149  0.000056 <= TS
[...]
  61     -1404.775014   -36.145667    0.005752  0.002294

Additionally, various useful files are written:
此外，还编写了各种有用的文件：

basename_IRC_F.xyz: endpoint of the forward run.
basename_IRC_F.xyz : 正向运行的终点。
basename_IRC_F_trj.xyz: trajectory of the forward run.
basename_IRC_F_trj.xyz ：前向运行轨迹。
basename_IRC_B.xyz: endpoint of the backward run.
basename_IRC_B.xyz : 反向运行的终点。
basename_IRC_B_trj.xyz: trajectory of the backward run.
basename_IRC_B_trj.xyz : 后退运行的轨迹。
basename_IRC_Full_trj.xyz: full trajectory of the complete IRC run.
basename_IRC_Full_trj.xyz : 完整 IRC 运行的全轨迹。

If we now look at the end-point of the backward IRC, we see that it the expected product of the oxidative addition or C-H activation of methane with the methyl and hydrogen substituents in cis arrangement in the equatorial plane of the octahedral product complex. The end-point of the forward IRC is the quadratic planar Vaska's type complex with a weakly coordinated methane molecule.
如果我们现在观察逆向 IRC 的终点，可以看到它正是甲烷氧化加成或 C-H 活化后预期的产物，其中甲基和氢取代基以顺式排列在八面体产物复合物的赤道平面上。正向 IRC 的终点则是具有弱配位甲烷分子的四方平面型 Vaska 型复合物。

../_images/irc-full.png — Figure: IRC path with molecular structures of the starting TS and the end-points.
图：包含起始过渡态和终点分子结构的 IRC 路径。#

Important 重要

The end-point of the IRC are not fully optimized structures and subsequent geometry optimizations should be performed to obtain the final reactant and product structures!
IRC 的端点并非完全优化的结构，应进行后续的几何优化以获得最终的反应物和产物结构！

Intrinsic Reaction Coordinate (IRC)#内禀反应坐标 (IRC)

Example: C-H Activation#示例：C-H 键活化

Structures# 结构

Intrinsic Reaction Coordinate (IRC)#
内禀反应坐标 (IRC)

Example: C-H Activation#
示例：C-H 键活化