Error in VMC force evaluation from bspline-wavefunctions of heterogenous systems

Error is found in the self-consistency assessment of VMC forces (ACForce) of CO in bpsline basis.

The bspline-wavefunction is very straightforward in terms of the ZVZB estimator, because its derivatives w.r.t. ionic coordinates are zero, that is, Fzvzb = Fhf. The account of implicit change of coefficients is more involved, but the self-consistency (correctness of the implementation) can be easily checked by fixing the wavefunction at eqm. and using it for all separations ("noJfix").

The trials were generated with QE (PBE0) with ccECP (aug-VTZ). No Jastrows were used, because they contain problems of their own (#1944 ).

The forces per ion should be equal to the energy derivative (i.e. F_1 = -F_2 = -dE/dR, where 1 and 2 refer to ions and R to their separation), but they are actually off by a large fraction. However, the errors are same in magnitude, leading to (F_1 - F_2)/2 = -dE/dR. This could be caused by a simple sign error in one of the force components.


Candidates for the bug can be narrowed down by a lot, because homogenous dimers (e.g. H2 and O2) do not show it.