Re: [Phys-L] simulation of "bonding"

[Jeff Spirko <spirko@gmail.com>]

I think John Denker's interpretation of the dependence on the initial
temperature is correct.  The reaction energy depends on the binding energy
and the energy required to get to/from STP.

On the trend toward higher total energy, does the time step affect this?
Change Ndisp and the delay at the same time as dt.  I'm on my way out the
door or I would fiddle with it.

It looks like the numerical technique is good.  It's the "symplectic Euler"
method, aka Euler-Cromer method that Bruce Sherwood said they kind of
stumbled across.  In each time step, velocities (here, momenta) are first
calculated based off the old coordinates.  Then all coordinates are updated
using the new velocities.

In the integrator, the effect of the electric force is ignored if the
electrons bounce.  I wonder if this introduces error in the energies.

If you ever expand to particles of different masses, the transform from lab
to C.M. frame must be adjusted.


--
Jeff Spirko   spirko@gmail.com   WD3V   |=>

The study of non-linear physics is like the study of non-elephant biology.

On Sun, Mar 6, 2016 at 8:06 AM, Robert Cohen <Robert.Cohen@po-box.esu.edu>
wrote:

> A couple more comments:
>
> 1. The programs don't display correctly in Explorer or Edge, as far as I
> can tell.
>
> 2. I added the values of the electric PE and the KE's of the pair and the
> environment.  The values are scaled by the difference in the initial and
> minimum PE's.
>
> 3. I created a new program where there is some initial movement in the
> particles (with the listed KE values being relative to the initial KE
> values).
>
> Compare:
> <
> http://www.glowscript.org/#/user/rcohenpa/folder/Public/program/chargeinteractions2
> >
> With:
> <
> http://www.glowscript.org/#/user/rcohenpa/folder/Public/program/chargeinteractions
> >
>
> 4. The electric force is 1/r because the region is 2-D.  Consequently, the
> PE is ln(r).  I don't think this makes a difference but I could do the F as
> 1/r^2 if anyone thinks that is a problem.
>
> 5. I notice that there is a slight trend toward higher total energy
> values, which I assume is due to the numerical technique but I'm not sure
> about that.
>
> ________________________________________
> From: Phys-l [phys-l-bounces@www.phys-l.org] on behalf of John Denker [
> jsd@av8n.com]
> Sent: Saturday, March 05, 2016 9:59 PM
> To: Phys-L@Phys-L.org
> Subject: Re: [Phys-L] simulation of "bonding"
>
> On 03/05/2016 06:58 PM, LaMontagne, Bob wrote:
>
> > I would love to hear other thoughts on this.
>
> My thought is that the simulation looks good to me
> in every detail.  It makes a fine lesson.
>
> Insofar as some people find some of the details
> surprising, it makes an even better lesson.
>
> >  the bonding energy is usually a fixed number.
>
> Chemists /report/ it as a fixed number because they
> habitually report the /standard/ enthalpy of formation,
> i.e. formation at (roughly) STP.
>
> If you care about the /actual/ (as opposed to standard)
> enthalpy of formation, it is undoubtedly temperature
> dependent, and this is nicely reflected in the simulation.
> The newly-formed pair appears to be in a vibrationally
> excited state (or the non-QM analog thereof) appropriate
> to the final temperature.  This looks kosher to me.
>
> If somebody sees something wrong or dubious, please
> explain in greater detail what you are seeing, and
> why it doesn't seem right.
>
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