# [CP2K:1303] Re: multiple force_eva run with different colvar restraints

Fri Sep 12 11:09:48 UTC 2008

```Hi Teo,

I am thinking to mix the restraints by colvars and a classical force_eval
section. Without including the colvar restraint, the mixed potential surface
is evaluated by the mixing function:
(1) E(mix)=(1-k)(E1+E3)+kE2 (E1 and E2 are the two end states, and E3 is the
restraint imposed by the classical force_eval).
As you said colvar restraints are applied at MD level, I think the overall
potential energy by adding the colvar restraint will be
(2) E(mix)=(1-k)(E1+E3)+kE2+V(r) (V(r) is the colvar restraint)
But there may be another possibility if we think that the colvar restraint
is imposed to each force_eval section individually. This will lead us to
(3) E(mix)=(1-k)(E1+V(r)+E3+V(r))+k(E2+V(r))=(1-k)(E1+E3)+kE2+(2-k)V(r)
which is different from the previous expression (Eq.(2)).
I tend to believe Eq.(2), but not 100% sure. What do you think?
Thanks.

Jun

On Wed, Aug 27, 2008 at 3:18 PM, Laino Teodoro <teodor... at gmail.com>wrote:

> Dear Jun,it is not possible since by definition restraints are applied at
> the MD level.. and your MD will be performed globally (i.e. on one system
> combination
> of the two states).
> What you want is to add maybe a classical term (like a bond) to your
> force_eval.. this makes everything more
> complicated but in principle feasible.
> good luck!
> Teo
>
>
> On 27 Aug 2008, at 15:55, Jun wrote:
>
> exactly the same restraints. Does anybody know how to specify the
>
> restraints separately?
>
>
>
> >
>
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