Talk:Constraint (computational chemistry)

Minor errors
Some (minor) errors seem to be present in formulas within the Lagrange multipliers section: I suppose that, instead of

This implies solving a system of n non-linear equations

$$   \sigma_j^{(t + \Delta t)} := \left\| \hat{\mathbf x}_{j\alpha}^{(t+\Delta t)} - \hat{\mathbf x}_{j\beta}^{(t+\Delta t)} + \sum_{k=1}^n \lambda_k \left[ \frac{\partial\sigma_k^{(t)}}{\partial \mathbf x_{j\alpha}}m_{j\alpha}^{-1} - \frac{\partial\sigma_k^{(t)}}{\partial \mathbf x_{j\beta}}m_{j\beta}^{-1}\right] \right\|_2^2 - d_j^2 = 0, \quad j = 1 \dots n $$ "

it should be

"This implies solving a system of n non-linear equations

$$   \sigma_j^{(t + \Delta t)} := \left\| \hat{\mathbf x}_{j\alpha}^{(t+\Delta t)} - \hat{\mathbf x}_{j\beta}^{(t+\Delta t)} + \sum_{k=1}^n \lambda_k \left[ \frac{\partial\sigma_k^{(t)}}{\partial \mathbf x_{j\alpha}}m_{j\alpha}^{-1} - \frac{\partial\sigma_k^{(t)}}{\partial \mathbf x_{j\beta}}m_{j\beta}^{-1}\right] \left(\Delta t\right)^2 \right\|^2 - d_j^2 = 0, \quad j = 1 \dots n $$ "

Al2bec (talk) 12:02, 22 April 2009 (UTC)


 * That is correct, the $$(\Delta t)^2$$ is missing. Good catch! Since it does not depend on the individual constraints, we could even move it out before the sum. In practice, it is usually left out since this constant scaling is absorbed by the individual $$\lambda_k$$.
 * Cheers,  pedrito  -  talk  - 22.04.2009 12:11

Some new developments
I think the recent articles that detail the effecient implementation of matrix inversion methods for linear and ring topologies

"Efficient constraint dynamics using MILC SHAKE" Journal of Computational Physics, Volume 227,  Issue 20  (October 2008) Pages 8949-8959

And a hybrid scheme for more general problems

MILCH SHAKE: An efficient method for constraint dynamics applied to alkanes Journal of Computational Chemistry Volume 30 Issue 15 (2009), Pages 2485 - 2493

should be mentioned —Preceding unsigned comment added by Cplowe1 (talk • contribs) 19:00, 16 December 2009 (UTC)

Summary is too general and not expository
I recently, changed the word mechanics in the first line of the summary to Physical Chemistry but, that is still too general. Also, there is no information linking this article to the constraints one is dealing with to give this article context. The summary says that contraint algorithms are to satisfy constraints. I think this is enough to remove the article altogether, as it has no link to other articles in wiktionary. The article on "contraint" will lead right back here from it's disambiguation page. Kckid (talk) 00:11, 15 September 2013 (UTC)

Missing mass term?
This article on the whole was quite well written. However, in working through the equations in the Lagrange section, I wonder if there is a missing mass term at the end of the section...


 * $$\hat{\mathbf x}_i(t+\Delta t) \leftarrow \hat{\mathbf x}_i(t+\Delta t) + \sum_{k=1}^n \lambda_k\frac{\partial \sigma_k}{\partial \mathbf x_i}$$.

... should this be ....?


 * $$\hat{\mathbf x}_i(t+\Delta t) \leftarrow \hat{\mathbf x}_i(t+\Delta t) + \sum_{k=1}^n \lambda_k\frac{\partial \sigma_k}{\partial \mathbf x_i}m_{i}^{-1}$$.

I'm not a maths expert, and I'm not fully sure if this is the correct solution, but without some sort of mass term in this final equation, then the same constraint acting on two different mass particles will result in the same movement of both particles, as if they were the same mass. Juggles82 (talk) 11:40, 25 December 2014 (UTC)

Requested move 7 February 2018

 * The following is a closed discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review. No further edits should be made to this section. 

The result of the move request was: page moved. TonyBallioni (talk) 17:17, 14 February 2018 (UTC)

Constraint algorithm → Constraint (computational chemistry) – May I suggest this article be renamed to "Constraint (computational chemistry)". That way the focus could be on the concept of a constraint with a section dedicated to various algorithms used to implement them. EvilxFish (talk) 14:31, 7 February 2018
 * Support, and with a bit more lay-language introduction (focusing on the idea not the specific applications). If we don't have an article about the whole topic, demonstrating notability of it (and I do think it's notable), I have trouble supporting having an article hosting a collection of ideas that are presented solely in the context of a non-notable idea. DMacks (talk) 16:54, 7 February 2018 (UTC)
 * Comment We also have a Constraint (classical mechanics) stub that is a bit of a mess IMO. DMacks (talk) 16:57, 7 February 2018 (UTC)
 * Comment Whereas I agree with the previous and would be happy to take a look at Constraint (classical mechanics) I think it is important to remember that there are unique features of constraints in the context of computational chemistry. For example the position of each particle is dependent on the position of other particles in the system (you can't keep a hydrogen atom in the same place if the carbon it is bonded to moves) and a discussion of how forces are modified to incorporate constraints. An explanation of why constraints are important and regularly employed in molecular dynamics simulations would be good. I would argue the notability of this subject is well established, it has a whole subsection in Leach's "Molecular modelling principles and applications" on pages 368-374 as well as pages 113-120 in the most recent edition of "Computer simulation of liquids". I know you acknowledge the notability I just wanted to make sure all other readers do as well.  EvilxFish (talk) 09:30, 8 February 2018 (UTC)


 * The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.