CASTEP，我一直以为自己找到了过渡态。。。。

xiaowandouer

用CASTEP也有段日子了，每次找过渡态，作业看似都正常结束了，各种输出文件都有，下面的图看起来也没什么毛病，于是每次都直接从.castep文件中取出能量去分析，可是今天取每个ts的坐标时却发现有些有，有些需要往上翻好几个scf循环的输出才能找到Coordinates: Fractional components，心生疑惑，把两种情况的输出文件对比了下，这才发现最后一步不给坐标文件的都有这样的提示：
QST cycle limit reached
QST cycle:                                       5
Maximum number of QST cycles:                    5
Transition state search failed
Please check the endpoints
Cumulative number of energy/gradient calls:    140


特别是看到过渡态搜索失败那一句，真的是扎心！我一直以为我找到了TS。。。。。。这种情况怎么办？结果能不能用？

xiaowandouer

杜伦大学的一篇博士论文中提到了这个问题的解决办法，不知道诸位是不是也这么做？
Although the algorithm had failed to find a transition state, it had identified some structures different to both the reactant and product, new geometries with the potential to be transition states, two examples of which are shown in Figure Figure 4.4. If these geometries were potential transition states, relaxing them to their ground states would allow them to fulfil this potential, although this would require strict convergence criteria as the slightest noise in the computations would cause a transition state to relax to a reactant or product. As the convergence criteria used for the transition state algorithm involved strict convergences, the same geometry optimisation criteria described in Section 4.3.2 were used to relax the QST maxima. The relaxed geometries show that one fell into the product configuration and hence was not a potential transition state but more of an intermediate on the route to becoming a product -- this is purely geometrical and has no chemical significance. One of the QST maxima relaxed into a completely new configuration, implying that a new minimum on the potential energy surface had been reached. At this point there was no way of differentiating between a true minimum and the minimum of a saddle point.

附链接：http://cmt.dur.ac.uk/sjc/thesis_dlg/node68.html

xiaowandouer

论文作者的导师是 Stewart Clark，CASTEP的开发者之一，是不是说明也没什么更好的办法了？关键是，就算重新优化QST maxima，得到一个新的构型，也不能确定这个到底是不是真正的过渡态。（At this point there was no way of differentiating between a true minimum and the minimum of a saddle point.）

我能想到的另一个办法也就是重新再找了。。。。。用过高斯的人表示CASTEP找过渡态太不灵活了