Reconstructing the wavefunction density from orbitals

[dotsdl]

Is your feature request related to a problem? Please describe.

Only eigenvalues and orbitals are currently stored in a Wavefunction object; however, we also need the density in order to rebuild the wavefunction in psi4 to calculate properties. So, the data to get the density is already present, but doing this calculation is left to each user that wants this, requiring domain knowledge that isn't exactly widespread.

Describe the solution you'd like

Wavefunction.density as a property or method that returns the density from the eigenvalues, orbitals.

Describe alternatives you've considered

This functionality may make more sense in QCEngine, but it is not clear yet.

Additional context

[hadim]

Jumping here as I was looking for an easy method to pull the density from a qcengine computation (using psi4).

I found it's possible to do that in psi4 using something like:

cube = psi4.core.CubeProperties(wfn)
cube.compute_properties()

but I don't know how to reconstruct the wfn object from the qcengine results result.wavefunction.

Also the psi4 snippet creates a file on disk, that would be nice to be able to extract the density in a python object (so letting the user the freedom to save it elsewhere).