请教有关DMOL3的设置问题
1. 用dmol3优化晶胞，想使晶胞的 xyz 轴夹角始终不变该怎么设置呢？help 里面说dmol不支持lattice parameters的固定啊！之前发现“use symmetry”打上勾可以保持xyz夹角不变，现在想请教大家如果不使用对称性，还有其他办法实现整个胞的夹角不变吗？
2. 关于dmol3加电场的问题。一般建立模型后直接run是不是默认没有电场？为什么和我 save files 之后加electric_field 0 0 0 算出来的总能不一样呢？想不通啊！
真诚请教有经验的虫友们，先谢谢大家了！
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1. 默认就是不优化晶格常数以及夹角的
2. 不知道
非常感谢！我计算时只要选中optimize cell，并且不加对称性，最后得到的晶格常数都发生了变化。不知道哪里出了问题
Opt_Constraint
Purpose
The lines following the Opt_Constraint keyword define constrained internal coordinates (bonds, angles, torsions) in molecules or solids. These internals are held fixed at the values of the initial structure.
Syntax
Opt_Constraint
constraint specifications ...
blank line
Description
For molecular structures the format is:
Opt_Constraint
i j k l
<repeat for next constraint>
<repeat for next constraint>
<etc...>
<blank line>
where the integers i, j, k, and l correspond to the numbers of the atoms in the .car file. The following designations are used:
i j 0 0: constrain the bond between atoms i and j
i j k 0: constrain the angle formed by atoms i, j, and k
i j k l: constrain the torsion formed by all 4 atoms.
For periodic structures the format is:
i ia ib ic j ja jb jc k ka kb kc l la lb lc
<repeat for next constraint>
<repeat for next constraint>
<etc...>
<blank line>
where i,j,k,l have the same meaning as for molecules, and ia ib ic, etc. indicate which unit cell contains the atom.
These constraints are added automatically when you run via the Materials Studio user interface. Use the Constraints option on the Modify menu to define constraints via the interfaces.
Example
In a molecule to freeze the bond between atoms 5 and 6, the bond between atoms 7 and 8, and the torsion 4-5-6-7 the input is:
Opt_constraint
5 6 0 0
7 8 0 0
4 5 6 7
<blank line>
In a periodic structure, to constrain the bond between atoms 1 and 2 in the same unit cell:
Opt_constraint
1 0 0 0 2 0 0 0 0 0 0 0 0 0 0 0
<blank line>
In a periodic structure, to constrain the bond between atom 3 in one unit cell and the periodic image of atom 4 one cell in the "a" direction:
Opt_constraint
3 0 0 0 4 1 0 0 0 0 0 0 0 0 0 0
<blank line>
To include a second constraint in the above example use:
Opt_constraint
3 0 0 0 4 1 0 0 0 0 0 0 0 0 0 0
2 -1 0 0 3 0 0 0 4 0 1 0 0 0 0 0
<blank line>
This constrains the angle, including the periodic image of atom 2 in the cell in the "-a" direction, atom number 3 in the reference cell, and the periodic image of atom number 4 in the "+b" direction.
将构XYZ三边的四个原子的相互角度固定可能实现你的目的。
2、加电场需要手动加关键词
electric_field
Purpose
The electric_field keyword allows you to specify the value of the external electric field used in the calculation. The field is described in terms of three cartesian components.
Syntax
electric_field value_in_X value_in_Y value_in_Z
command keyword values default meaning
electric_field real real real 0.0 0.0 0.0 The x, y and z components of the electric field applied to the system.
Example
This example is for electric field with strength 0.05 a.u. applied in the Z direction only:
electric_field 0.000 0.000 0.0500，
非常感谢您的耐心回复！对我挺有帮助！可还有些地方不太清晰：
第一个问题，最后您建议固定xyz三边的四个原子间的角度，很受启发。可我现在的模型是一个单层薄片，在Z方向（垂直薄片方向）加的真空层，该怎么保证Z轴垂直，还是有点迷惑。
第二个问题，我也是按您说的步骤手动加的关键词，问题是当我加0 0 0电场时和直接优化不加电场所得的体系总能不一致，本来0电场和不加电场应该效果一样才对啊。
再次表示感谢了！
选中optimize cell就是对晶格参数和原子位置同时进行优化，不选则只优化原子位置。
恩，是的！之前不选optimize cell计算了几次都失败了，我再调整下参数试试！
第一个问题，最后您建议固定xyz三边的四个原子间的角度，很受启发。可我现在的模型是一个单层薄片，在Z方向（垂直薄片方向）加的真空层，该怎么保证Z轴垂直，还是有点迷惑。
只有一层的时候哪怕优化过程中C轴与A、B轴的夹角发生变化不再垂直，这个只是看起来不好看，但是对问题本身似乎是没影响的，因为你保证相邻层之间足够远，没有相互作用，所以C轴没什么意义，你可以计算完了在写论文作图的时候再把它做成垂直的。