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小木虫论坛-学术科研互动平台 » 计算模拟区 » 第一性原理 » Siesta&Smeagol&Atk » 【讨论】Nanolanguage Scripter 中的设置 疑惑
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fan0221

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[交流] 【讨论】Nanolanguage Scripter 中的设置 疑惑

建立好模型后 在Nanolanguage Scripter可以进行具体参数设置 Method 下有很多参数 其中 Brillouin Zone Intergration 中有三个参数 Number of k-points
(A),(B),(C).设置完之后在Analysis中计算 Current Density of states Transmission spectrum 时  又出现了Number of k-points
(A),(B)参数设置的选项
这里的参数的含义是什么  与Method下的有什么异同  可以用统一的参数吗
例如 我前面是 3，3，100  这里写 3， 3？
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1楼 2009-03-17 12:35:12
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fan0221

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没有人关注啊
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2楼2009-03-17 19:07:47
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acridine

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zdhlover(金币+1,VIP+0):谢谢 3-20 23:22
一般的软件好像都是这样，自洽计算使用一组K点设置，性质计算（如dos图或这里的电流）时可以使用另一种设置
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3楼2009-03-18 08:09:27
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fan0221

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qasd(金币+3,VIP+0):期待多讨论 3-18 20:14
呵呵  在QW也问了同样的问题  原来第一个是电极scf计算第二个是电极和分子的偶合计算  具体内容放出来  感兴趣的看一下吧 版主要是觉得好 记得给点币币
本人认为这个参数关系到最后计算结果的合理性 应该重视一下

The first setting, for the self-consistent calculation (SCF), is related to the band structure of the underlying electrode bulk crystal. To determine the Fermi level accurately and to sample the band structure properly, a certain number of k-points are needed. For a small unit cell (small in the XY directions that is), you need more k-points, perhaps up to 10x10 for good accuracy, while if you have a large unit cell you can perhaps even get by with a single point (1x1).

The second "analysis" settings determines the sampling of the 2D Brillouin zone when you calculate the transmission spectrum. Electrons incident from the elcetrode with different k-points have different probability of tunneling through the central region, and we must sum all contributions to obtain the total transmission at a certain energy. In this case it can be very hard to know up front how many points are needed, and one should really test by increasing them to see if the transmission spectrum changes.

The number of k-points do not have to be the same. If we take a well-known example of a FeMgO magnetic tunnel junction, an SCF sampling of 8x8 is sufficient for good results. However, the minority transmission spectrum for the parallel spin configuration requires upwards of 100x100 to reach a good value for the transmission. This is because there is large number of very narrow peaks in the transmission coefficients, which are related to the exact matching of the Fe band structure to the MgO energy levels.

Therefore, as we see in this example, the analysis k-points are related to the whole system, the transmission properties of the composite two-probe system, while the SCF k-points are only related to the electrode.

The only way to really find out how many k-points are needed to obtain converged results, is by running a sequence of calculation for different values. This goes for both the SCF and analysis k-points, although the transmission spectrum typically (but not always!) is less sensitive to the SCF k-points.

Increasing the number of k-points in the SCF does however improve the convergence, and if there are way too few k-points (e.g. using only Gamma point when one should have 5x5 instead) will shift the band structure and thus influence the results.

ATK is specifically parallelized over k-points, so when running on a cluster, it is not very expensive to increase the k-point sampling, if you run on many nodes. Memory usage does, however, increase with the number of k-points.

Of course, for one-dimensional systems like nanotubes, you always only need 1x1 k-points for both SCF and analysis. For graphene, which is periodic in two directions, you need N points in one direction and 1 point in the other (perpendicular to the sheet).
转自 QW  感谢 Anders Blom
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4楼2009-03-18 09:42:28
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