Re: [ESPResSo] ESPResSo Digest, Vol 39, Issue 14

[Steven R. Kirk]

> On Monday 23 August 2010 09:39:09 Steven R. Kirk wrote:
>> Hello!
>>
>> I'm using tabulated potentials for calculating interactions between
>> particles of a single species. I want to alter this potential mid-run,
>> and I also want change the number of points where the new potential is
>> tabulated (currently I get the error message:
>>
>> number of data points does not match the existing table
>>
>> )
>>
>> So can I do the following:
>>
>> inter 0 0 tabulated tab_pot1.dat
>>
>> Then later in the script do:
>>
>> inter 0 0 tabulated tab_pot2.dat
>>
>> ?
>> Do I need to remove the first interaction before specifying the second?
>> Are there any alternative means to achieve the same goal (if so, would
>> be very grateful for details).
> 
> Hi!
> 
> I am not using tabulated potentials myself, but a quick scan of this code 
> shows that there is currently no way to change the number of points. The 
> reason is, that there is one table, in which all the tabulated values are 
> allocated, and if you would want to increase the number of points in one of 
> the stored potentials, then all the potentials stored afterwards would need 
> to 
> be shifted, which simply isn't implemented and actually not so easy.
> 
> For your suggestion to delete the interaction: there is no way to delete an 
> interaction. There is a table of interaction parameters for pairs of 
> particles 
> of certain types. Switching off a particular interaction simply means to set 
> its cutoff to 0, so that it is not used for this specific pair of types. But 
> there is no way to change the number of points in a tabulated interaction 
> back 
> to 0.
> 
> Axel
> 

Many thanks for your help Axel!

How would I set the cutoff for a particular interaction to zero, once it
has been loaded? The documentation for the inter command says that the
outer cutoff for a potential is set by the maximum distance specified in
the tabulated data.

So once I call:

inter 0 0 tabulated tab_pot1.dat

then the only way I can change the cutoff is by, for example, later
issuing the command

setmd max_range 0.0

Is this correct? If so, then I still can't switch from one potential to
another in mid-run, since presumably max_range applies to *all*
currently loaded interactions?

As an alternative way to achieve my goals, would it be possible to:
1. Save a checkpoint after the first phase (i.e. running with the first
tabulated potential) that does not have the interaction information
recorded in it, but contains all other parameters (positions +
velocities+ temperature+ thermostat+ .....)
2. Close Espresso
3. Restart Espresso and immediately load in the checkpoint file
4. Define the new interaction by loading the new tabulated interaction
data from a file
5. Continue integrating using the new potential

I would be very grateful if you could confirm/refute this possibility.
If you think it would work, could you suggest the syntax needed for the
command needed to create and save the checkpoint file?

Many many thanks,
Steve Kirk