[CPMD-list] Re: two questions about MD

[qfzhang]

Hi,professor!
   Thanks for the reply!
   I learned from some tutorial that the average force can be calculated using d
F/dq=<Z-1/2[-lamda+kTG]>/<Z-1/2>, and what's the quantity in the CONSTRSINT file
? Is it lagrange multiplier lamda?

   best wishes
                                                  Zhang Qianfan
                                                   2007-02-28
   

Axel Kohlmeyer д:

> On Sun, 25 Feb 2007, qfzhang wrote:
> 
> dear zhang,
> 
> ZQ> Hi,professor
> ZQ>   Thanks for the reply! I still have some question about the constraint fo
rce.
> ZQ>   Do you mean that the constrain force of one particular distance differen
ce is 
> ZQ> just the average of the trajectory, and the barrier can be calculated just
 by in
> ZQ> tegrating such \'averages\'?
> 
> that is correct, provided the your system is in equilibrium.
> please check out the literature about umbrella sampling and
> statistical mechanis in general. there are some references 
> in the manual and most modern textbooks on MD cover it decently.
> there are also a number of alternatives to umbrella sampling 
> (which is considered to be rather accurate - if done correctly, 
> but quite expensive in terms of cpu time requirements).
> 
> ZQ>   Another problem is that the constraint forces in the CONSTRAINT file var
y heav
> ZQ> ily along the trajectory. Is there some method to solve such a problem? Ge
nerall
> ZQ> y speaking,how many timestep should be done to get the converged constrain
t forc
> ZQ> e? 
> 
> this is to be expected. the instantaneous forces should fluctuate in a 
> more-or-less gaussian distribution around the \'true\' values and then
> you can use conventional statistical measures to judge whether your 
> average is converged or not. you also have to balance the accuracy 
> of each sampling point against the accuracy of integrating the mean 
> forces with a limited number of sampling points. of course the number 
> of sampling points depends on the shape of your PMF.
> 
> ZQ>   From the example given in cpmd tutorial, when specifing CONSTRAINTS, the
re is 
> ZQ> no NOSE keyword, should that be needed in the constant temperature simulat
ion?
> ZQ>    best wishes 
> 
> can you please be more specific to which example of what tutorial you 
> are referring to. 
> 
> cheers,
>    axel.
> 
>