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In the Simulating Molecules using VQE section of the Qiskit textbook it states an accuracy of $0.0016$ Hartree between the exact and VQE results with Hartree energy values of $-1.86712098$ and $-1.80040360007339$ for the ground state of $H_2$, respectively.
How is $0.0016$ computed? and is it an error metric similar to absolute error?
I think you may have misread the section - the document says:
When noise mitigation is enabled, even though the result does not fall within chemical accuracy (defined as being within 0.0016 Hartree of the exact result), it is fairly close to the exact solution.
So, the VQE result is not within chemical accuracy, but it is fairly close to chemical accuracy.
How is this number derived? As stated on Wikipedia:
Chemical accuracy is the accuracy required to make realistic chemical predictions and is generally considered to be 1 kcal/mol or 4 kJ/mol. To reach that accuracy in an economic way it is necessary to use a series of post-Hartree–Fock methods and combine the results.
So, if you convert kcals to Hartrees and mols to molecules, you yield about 0.00159 hartrees/molecule, exactly what IBM says.
