[CPMD-list] how to do constraint optimization?

[Lan-Feng Yuan (lyuan@Princeton.EDU)]

----- Original Message -----
From: Salomon Billeter <ZRLSRB at ch.ibm.com>
Date: Friday, December 9, 2005 6:13 am
Subject: Re: [CPMD-list] how to do constraint optimization?
To: "Lan-Feng Yuan (lyuan at Princeton.EDU)" <lyuan at Princeton.EDU>
Cc: cpmd-list at cpmd.org

> Dear Lan-Feng,
> 
> version 3.10 is not affected by the bug. With the linear-scaling 
> geometryoptimizer, the optimization will converge quickly (it 
> converged quickly
> here, while the conjugate gradient optimization did not converge 
> either).To switch to L-BFGS, change the keyword 'CONJUGATE 
> GRADIENTS IONS' to
> 'LBFGS' in the &CPMD section. You best also switch on diagnostics 
> usingPRINT LSCAL ON.
> 
> May I comment on two other portions of your input:
> - You have an isolated system but not SYMMETRY=0. Your periodic 
> images will
> be electrostatically coupled. Also, your cell size is rather at 
> the lower
> limit.
> - You have very tight convergence criteria, espeically for the 
> electrons.If you start with a large nuclear gradient (as you do), 
> adaptive tolerance
> saves you many cycles (keywords CONVERGENCE ADAPT, CONVERGENCE 
> INITIAL, and
> - in your case probably not too helpful - CONVERGENCE ENERGY, 
> CONVERGENCECALFOR, and CONVERGENCE RELAX). Values around 0.01 and 
> 0.0001 for the first
> two options could speed up the calculation in your case.
> 
> When reaching convergence, please make sure the constraint is 
> maintainedaccurately enough. As written before, CPMD does not 
> determine Lagrange
> multipliers during geometry optimization for most optimizers, and the
> penalty could be too small.
> 
> Best regards,
> Salomon
> 
  Dear Salomon,

  Thank you very much. Adopting your suggestions, I indeed get convergence. Well, the output says:

                     <<<<< CONSTRAINTS >>>>>
 FIXED STRUCTURE ELEMENTS
      TYPE     ATOM ATOM ATOM ATOM                VALUE      DIFFERENCE
  DISTANCE       1    2    0    0               5.57361   -1.086070E-03

  Therefore, the constraint is not exactly fulfilled, leaving a small difference (0.001 Bohr). I have no complaint on that. But is there any way to select the threshold of tolerance? Thanks! Best regards,

  Lan-Feng