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奋力拼搏至尊木虫 (文坛精英)
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[求助]
请问用CASTEP如何计算磁性物质FeVSb的单点能？谢谢！！
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| 用MS5.5中的CASTEP计算FeVSb的单点能；由于FeVSb是磁性物质，请问应该如何设置？十分感谢！！！渴望得到高手的指教！！！ |
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2楼2013-07-17 17:26:01
奋力拼搏
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3楼2013-07-18 10:10:19
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【答案】应助回帖
★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★ ★
奋力拼搏: 金币+20, ★★★很有帮助, 如果我把合金中各个磁性原子的 spin值输入后，Use formal spin as initial spin就可以不用勾选了？下面的Initial spin和charge如何处理呢？谢谢您了 2013-07-18 15:04:48
csgt0: 金币+2, 谢谢 2013-07-18 17:55:02
奋力拼搏: 金币+20, ★★★很有帮助, 如果我把合金中各个磁性原子的 spin值输入后，Use formal spin as initial spin就可以不用勾选了？下面的Initial spin和charge如何处理呢？谢谢您了 2013-07-18 15:04:48
csgt0: 金币+2, 谢谢 2013-07-18 17:55:02
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勾选Use formal spin as initial即可，表示用系统默认的数值来作初值；不勾选的话，需要自己输入Initial spin的数值，Fe的话，2.0作初值就可以了。charge不管，你的体系有没有带电荷，默认值就可以了。 Electronic——more——SCF——Density mixing中的charge和spin只有当你把Electronic minimizer的算法选为Density mixing才会有，这样的charge和spin是Density mixing算法中的参数，系统默认值一般即可。 |
4楼2013-07-18 11:30:07
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5楼2013-07-18 15:13:58
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6楼2013-07-18 15:16:32
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7楼2013-07-18 15:58:57
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8楼2013-07-19 08:39:08
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★ ★
csgt0: 金币+2, 谢谢 2013-07-19 14:52:53
csgt0: 金币+2, 谢谢 2013-07-19 14:52:53
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如果自己指定初始值的话，应该是每个晶胞中的总数 (per unit cell)。 CASTEP提供了两种定义磁性的途径，你现在打算采用的是第一种，如下： Spin polarization Magnetic systems can be studied by carrying out spin-polarized DFT calculations with CASTEP. This setting is recommended for transition metal oxides, certain inorganic surface studies, and metallic systems containing magnetic elements (Fe, Co, Mn, Ni). Spin-polarized CASTEP calculations are normally carried out with a variable magnetic moment. This procedure, however, does not guarantee finding the lowest energy state. DFT solutions can converge to a variety of local minima, so that the metastable states are found. The most commonly obtained solutions correspond to high spin and low spin states. The state generated by CASTEP depends on the initial magnetic configuration, since the solution is likely to converge to the nearest local minimum rather than to the global minimum. There are two ways of defining the initial magnetic configuration: either specify the total magnetic moment per unit cell, which gets uniformly distributed over the space, or provide detailed information on the absolute values and direction (up or down) of the spins for each atom in the unit cell. The former method can be used for relatively simple systems where only two solutions are expected (magnetic and non-magnetic). The latter method, which specifies the spin state of the atoms in the system, is more general and gives much more flexibility. It is possible to set up ferromagnetic, ferrimagnetic, or antiferromagnetic calculations to get different starting spin arrangements. To set up a spin-polarized calculation with a uniformly distributed initial magnetic moment Choose Modules | CASTEP | Calculation from the menu bar to display the CASTEP Calculation dialog. Select the Setup tab. Check the Spin polarized checkbox and uncheck the Use formal spin as initial checkbox. Specify an Initial spin value. To optimize the spin value, select the Electronic tab and click on the More... button to access the CASTEP Electronic Options dialog. Select the SCF tab. Uncheck the Fix occupancy checkbox and check the Optimize total spin checkbox. To set up a spin-polarized calculation with a specified ordering of atomic moments Note: Initial magnetic moments on atoms are treated correctly by CASTEP only when the density mixing scheme is used for electronic structure minimization. CASTEP will ignore atomic information and will use the uniformly distributed magnetic moment for the all bands or EDFT minimizers. The value of the total moment is determined by the sum of formal spins in the system. Set formal spins for the atoms. After setting the formal spins, find and impose the symmetry of the system. Choose Modules | CASTEP | Calculation from the menu bar to display the CASTEP Calculation dialog. Select the Setup tab. Check the Spin polarized and Use formal spin as initial checkboxes. Select the Electronic tab and click on the More... button to access the CASTEP Electronic Options dialog. Select the SCF tab. Select Density mixing as the Electronic minimizer setting. Uncheck the Fix occupancy checkbox and check the Optimize total spin checkbox. It is important to include a sufficient number of empty bands when optimizing the spin state. The default number of empty bands is 4, which means that the maximum accessible increase in the spin value is 8. A calculation that started with the low spin state will be extremely slow if the actual minimum found by CASTEP corresponds to the high spin state (although this is not the case when a calculation starts with high spin and converges to low spin). Note: CASTEP might fail to converge the SCF if the number of empty bands is insufficient to accommodate the high spin state, especially in a calculation with variable occupation numbers (metallic systems). 建议你仔细学习一下CASTEP的help文档，里面的解释比较权威。 |
9楼2013-07-19 08:54:40
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10楼2013-07-19 15:22:03
