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Dear Casino users (and especially Neil)!
Could you please give me any hints on some questions concerning the example distributed within input/exmol directory?
1. Firstly, have you published these (or similar) results produced by Casino, especially concerning typical benchmarks for positron systems: Ps2, Ps-, PsH etc. and including any related technical details? What references could you advice on this?
2. You have used the H term in the Jastrow factor which is not documented in casino manual (at least I have not found anything). I have only realized that this is some three-body term which somehow takes into account the information from %block custom_striplet_dep.
I wonder how does this term look like and how does the input block above enters its calculation?
3. I have done the DMC simulation based on your example. Here is the plot of binding energy vs. time step. (I have tried the calculations without the mysterious H term in Jastrow and without backflow transformation). It seems that the H term is quite useful for calculations, while the backflow correction is not. Am I wrong? What do you think about this?

Dear Dmitry,

To my knowledge no results on Ps2, etc., have been published using exmol. It was implemented by Pablo and you might want to ask him about this...

The H term is a polynomial three-body Jastrow term, a bit like the f term (which is a three-body term for two electrons and an ion). The custom_striplet block lets you define the types of particle triples that have the same or different three-body terms. In a Ps2 molecule there is only one kind of triple: triples are either two electrons and a positron or two positrons and an electron, and these should have the same three-body terms. If you were looking at a biexciton with different electron and hole masses, there would be two types of triple: two heavy particles and light particle or two light particles and a heavy particle. However you would also need to define a "null" spin-triple type (for the non-exist electron-electron-electron triples, etc.). Something like

%block custom_striplet_dep
no_striplet_deps 1
striplet_dep -1 3
1=1=1,2=2=2,3=3=3,4=4=4,1=1=2,1=1=3,1=1=4,2=2=1,2=2=3,2=2=4,3=3=1,3=3=2,3=3=4,4=4=1,4=4=2,4=4=3 # Unused triples
1=2-3,1=2-4 # Two electrons and a hole
3=4-1,3=4-2 # Two holes and an electron
%endblock custom_striplet_dep

You can then use this by setting the spin-dependence to -1 in the H term.

The systems you are looking at do not have nodal surfaces. The node-altering abilities of backflow are not helpful, although backflow can provide some description of three-body correlations.

Best wishes,

Neil.