几何限制（constraint，这个词很多时候在中文翻译上和restraint有所混淆，都经常被翻译成“约束”。个人觉得翻译成"限制"比较好，因为它有不留余地的含义，而“约束”则表示还有活动的余地，只是被控制在一定范围内）在模拟中有着特殊价值，例如我们需要保留分子结构中关键部位的构型不变，但是用模拟方法优化其他部分，这是最好的办法就是使用限制。但是从算法角度讲，几何限制涉及到复杂的方程组求解问题（限制方程），或者影响到体系的边界条件，需要仔细处理，在不同的模拟算法中实现起来有不同的难度。最常见的几何限制就是对化学键键长的限制，广为使用的是SHAKE和RATTLE方法。更高级的限制在MS中也有部分支持，例如对晶格对称性的限制，对原子团的限制，对两面角的限制等。在MS中，对于分子模拟方法来说，可用的限制方法并不如想象的多。常用的discover和forcite基本上只支持原子位置和键长限制，Forcite中只有几何优化支持motion group。所以说实话还是有很多情况下不够用。对于每种方法对限制条件的支持，手册中有如下文字，摘录于此供参考：
The modules that currently support constraints are listed below, along with details of the types of constraints that are supported:
CASTEP: Supports atom positions fixed in Cartesian space, fixed lattice parameters, and fixed interatomic distances, angles, and torsions, but ignores all combinations of constraints on the x, y, and z components of Cartesian atom positions and geometric constraints.
Discover: Supports atom positions fixed in Cartesian space, but ignores partial constraints on the x, y, or z components of Cartesian atom positions, constraints on fractional positions, and geometric constraints.
Note. It is not possible alter constraints between simulations when using the Restart option.
DMol3: Supports atom positions fixed in Cartesian space, and partial constraints on the x, y, or z components of Cartesian atom positions, but ignores constraints on fractional positions and lattice parameters. Additionally, DMol3 supports fixed interatomic distances, angles, and torsions in nonperiodic structures.
Forcite: Supports atom positions fixed in Cartesian or fractional space and fixed lattice parameters, but ignores partial constraints on the x, y, or z components of Cartesian atom positions and geometric constraints.
GULP: Supports atom positions fixed in Cartesian or fractional space, but ignores constraints on lattice parameters, partial constraints on the x, y, or z components of Cartesian atom positions, and geometric constraints.
ONETEP: Supports atom positions fixed in Cartesian space and fixed lattice parameters, but ignores all combinations of constraints on the x, y, and z components of Cartesian atom positions and geometric constraints.
Note. ONETEP cannot change lattice geometry.
Reflex/Reflex Plus (Powder Refinement and Powder Solve): Supports fixed interatomic torsions and fixed lattice parameters, but ignores constraints on Cartesian and fractional atom positions and other geometric constraints.
Note. All the other Materials Studio modules ignore all the types of constraints that can be defined using the Edit Constraints dialog.
其他没有在上面提及的模块，那就是一点也不支持限制条件了，不要白费力气了。我曾多次尝试过在md中限制特定两面角，均告失败。看来那个功能暂时只是给Dmol设计的。

谢谢你的总结。

总结的很好！