Re: [Cppqed-support] Position correlation

[Andras V. <and...@ui...>]

Hi Raimar,

In this case, your *LazyDensityOperator* will be binary only, which means
that it can be indexed with 4 indices (2 instances of *TTD_IDXTINY(2)* ).
You have to copy the whole thing into a *
quantumdata::Types<2>::DensityOperatorLow* . This is actually rather
wasteful, because even in the case when the underlying data is just a state
vector, it is treated in the same way as a full density operator. You could
maybe think about a nice general solution to this problem (I have just
added a tracker #3487814 on this issue).

Anyway, once you have your *DensityOperatorLow*, I think you have to apply
FFT on *all of its indices individually,* the FFT's sign depending on
whether it is a ‘left’ or ‘right’ index. So, *
blitzplusplus::vfmsi::fullRange* will not be of much use in this case, and
you have to apply the more general
*blitzplusplus::basi::fullRange*iteration. I think the code is
something pretty much like this:

// left:
for_each(fullRange(rhotemp,Vector<0>()),bind(&ffTransform,_1,FFTDIR_KX));
for_each(fullRange(rhotemp,Vector<1>()),bind(&ffTransform,_1,FFTDIR_KX));
// right:
for_each(fullRange(rhotemp,Vector<2>()),bind(&ffTransform,_1,FFTDIR_XK));
for_each(fullRange(rhotemp,Vector<3>()),bind(&ffTransform,_1,FFTDIR_XK));

Where *Vector* stands for *tmptools::Vector*. (This is not the only class
that works with the slice iterators, because e.g.
*boost::mpl::range_c*also works, but here you need this.)

Keep in touch. Best regards,
András