Dear Mike and Neil,
I actually have some concern about using the T-moves implemented in CASINO in large systems.
As far as I understood (and please correct me if I am wrong) the T-move algorithm implemented in CASINO is the one introduced in by Michele Casula in its first PRB 2006 paper (
http://dx.doi.org/10.1103/PhysRevB.74.161102), with the only difference that the branching is taken to be symmetric.
However, Michele and others have later observed a size-consistency issue in the first T-move algorithm, as addressed in this paper:
J. Chem. Phys. 132, 154113 (2010)
http://dx.doi.org/10.1063/1.3380831
The problem is (quoting their JCP paper) that:
"for given time-step, the probability of a
successful move will increase with the system size i.e., the
number of electrons and saturate to one for sufficiently large
systems. In this limit, the effect of the move will become
independent of the system size and lead to one electron being
displaced at each step. Therefore, for sufficiently large systems,
the overall impact of the nonlocal move will decrease
and the algorithm will effectively behaves more and more
like in the LA procedure."
I bet that in large systems nobody will use very small time-steps, thus I guess that the T-move implemented in CASINO will behave as the "DMC Ref. 7 sym" method shown in the aforementioned JCP paper, Fig.2(a).
In the JCP paper they provide two algorithms that would solve this problem, called SVDMC Version 1 and Version 2, neither of which seems to me implemented in CASINO.
Given that, I think it would be better to use the locality approximation in big systems.
Is there anyone willing to implement the any one of the SVDMC T-move algorithms in CASINO?
Best wishes,
Andrea