Add HexagonalLattice Class

[javabster]

Summary

fixes #956

Details and comments

[coveralls]

Pull Request Test Coverage Report for Build 4053980683

• 20 of 22 (90.91%) changed or added relevant lines in 2 files are covered.
• 73 unchanged lines in 4 files lost coverage.
• Overall coverage increased (+0.07%) to 85.963%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_nature/second_q/hamiltonians/lattices/hexagonal_lattice.py	19	21	90.48%

Files with Coverage Reduction	New Missed Lines	%
qiskit_nature/second_q/properties/electronic_density.py	2	98.39%
qiskit_nature/second_q/drivers/pyscfd/pyscfdriver.py	7	75.86%
qiskit_nature/second_q/mappers/qubit_converter.py	10	93.68%
qiskit_nature/second_q/algorithms/excited_states_solvers/qeom.py	54	81.13%

Totals
Change from base Build 3901930559:	0.07%
Covered Lines:	17693
Relevant Lines:	20582

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💛 - Coveralls

[mrossinek]

Besides these comments, please also add a release note to this PR

[woodsp-ibm]

Do we want to include this in the Lattice Models tutorial - since that tutorial shows the existing models it seems to me it would be good to include this there too.

[mrossinek]

Yesterday I found out that networkx supports both: periodic boundary conditions and the computation of the real-space coordinates of a regular hexagonal lattice structure (something which we like to use to make the graphs look predictable; see for example here).

@mtreinish is rustworkx open to supporting similar functionality? At least periodic boundary conditions would seem useful to me.

If yes, I suggest we could track the addition of similar functionality there. Since I moved this PR to 0.7.0 we could even postpone until that is available. @javabster wold you be interested in contributing this to rustworkx, assuming they want it 😄

[mtreinish]

Yeah, both of those seem totally valid to add to rustworkx. If you don't mind can you open an enhancement issue:

https://github.com/Qiskit/rustworkx/issues/new?assignees=&labels=type%3A+enhancement&template=ENHANCEMENT_REQUEST.md

with the details and we can target it for the 0.13.0 release (which we don't have a target date for yet). @javabster it'd be great if you're interested in contributing the functionality to rustworkx too. I can give you a hand if you need any help getting started on it.

[javabster]

issue opened: Qiskit/rustworkx#803

[mrossinek]

For various bandwidth reasons I would like to restrict the scope of this PR as follows:

• we neglect periodicity for the time being and instead add support for this in the future (separate issue to be opened)
• we need to add a computation of the default node positions (to ensure consistent and nice visualization; see for example the networkx implementation of this)
• in the future, rustworkx might do this last point directly at which point we can remove this from this class. Thus, this computation should be done in a private method which we can remove without API breakage

[CLAassistant]

All committers have signed the CLA.

[mrossinek]

Let's also get this one merged. I added a comment here to keep track of possible future extensions.

I also added a simple release note in a similar style to the one of the recently merged Kagome lattice.

Thank you, Abby, for contributing to Qiskit Nature and being so patient with this lengthy (and sometimes stalled) review process!