[gmx-developers] Implementing Monte Carlo rejections in md.c

[Grant Rotskoff]

Hi John,

Which thermostat are you using?

You may try calling the update_randomize_velocities routine in
update.c. This uses an Andersen thermostat and is present in md.c, so
there may be an example that you can take inspiration from.

Grant.

On Mon, Feb 24, 2014 at 9:24 PM, j_haberstroh <john.haberstroh at gmail.com> wrote:
> Hi Developers!
>
> I'm working on including a MC step into my MD that conditionally accepts a
> step with some probability that I compute from the difference between the
> initial and final coordinates. This part is all fine and dandy, I'm getting
> appropriate computations and appropriate probabilities.
>
> I must do something beyond the simplest rejection of moves: I cannot simply
> roll back x and v and expect my system to sample new positions; since the
> eqns of motion are deterministic, I will never sample new positions, and the
> MC will not produce any useful effect.
>
> My approach right now is to save the positions and velocities into new
> variables that I've snew'd called xumb and vumb, and then to write them back
> to the state->x and state->v with copy_rvecn(). I do this MC step right
> after the end of END UPDATE STEP 2. Then I want to scramble the velocities a
> bit, probably by tcouple-ing.
>
> My issue is that my system is totally indifferent to the value I set to
> state->v. I've tried setting state->v to zero on rejection, tried
> thermalizing at the beginning of the loop, at the end of the loop... nothing
> I do seems to change the final position that I end up in.
>
> Here's a little sample of some of my output:
>
> dE_0=-57.107506...Step 29...dE_f=-57.140678...  Change in dE=-0.033173
> dE_0=-57.107506...Step 30...dE_f=-57.140678...  Change in dE=-0.033173
> dE_0=-57.107506...Step 31...dE_f=-57.140678...  Change in dE=-0.033173
> dE_0=-57.107506...Step 32...dE_f=-57.140678...  Change in dE=-0.033173
> dE_0=-57.107506...Step 33...dE_f=-57.140678...  Change in dE=-0.033173
> Accepted!
> dE_0=-57.140678...Step 34...dE_f=-57.187866...  Change in dE=-0.047188
> Accepted!
> dE_0=-57.187866...Step 35...dE_f=-57.242420...  Change in dE=-0.054554
> Accepted!
> dE_0=-57.242420...Step 36...dE_f=-57.296158...  Change in dE=-0.053738
>
> Steps 29-33 are rejected over and over, and the final dE does not change
> even though I have an additional update_tcouple() after each rejection.
> Either something is conspiratorially abyssmal or the code is somehow
> circumventing my attempts to brutally modify it.
>
> Any tips? I want to pull a thermal ensemble of velocities.
>
> Thanks!
> John Haberstroh
>
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