Re: [ESPResSo-users] Individual particle energies

[Rudolf Weeber]

Hi,
On Tue, Dec 04, 2018 at 12:53:17PM +0000, Rhys Goodall wrote:
> I'm looking to find the potential energy in order to use Henderson's method 
> for estimating the cavity function. What do you mean by specific energy 
> contribution?
> 
> This boils down to evaluating psi = sum_{i != t}  phi(r_i) where phi(r_i) is 
> the potential energy between the specified particle (t) and the ith particle. 
> I supposed that I can pull the particle positions and then evaluate the 
> relevant potential energies from my interaction tables if there's not a 
> convenient way to do it already implemented.
This information is not directly available in Espresso.
An easy but not efficient workaround would be to temporarily remove the 
particle and compare the difference in energy.

E_with =system.analysis.energy()["total"]
p=system.part[id]
id,pos,v,q,type=(p.id,p.pos,p.v,p.type)
system.part.remove(id)
E_without =system.analysis.energy()["total"]
system.part.add(id=id,pos/pos,v=v,type=type)

You might have to store more particle properties, and if bonds are involved, 
things are more complicated.

You'd have to try whether this meets your performance requirements.

A less drastic solution might be to temporarily alter the particle type and 
charge (again ignoring bonds)

Regards, Rudolf