PHphilipkglass·Dec 29, 2018
Standard quantum chemical methods for this sort of problem would finish in a fraction of a second on a Raspberry Pi. Calculating the ground state energy of a tiny system like LiH was tractable way back in the 1960s. I'd need to see their actual numbers to determine when a conventional computer first reached their level of accuracy on LiH but I'm sure it is several decades back.
EDIT: according to the paper, the initial energy at each point was found using the Hartree-Fock method with a minimal STO-3G basis set. This is one of the simplest and oldest approaches to this sort of calculation on a conventional computer. For these starting calculations they used Psi4 [1] by way of OpenFermion [2]. For the H2 molecule, their additional DWave calculations improved the accuracy of the distance-energy curve over the baseline Hartree-Fock/STO-3G cal
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culations. For LiH, there was no improvement (Figure 3). The total runtime of their approach was therefore that of the conventional approach plus an additional series of calculations that did not yield improvements in the case of LiH.
[1] http://www.psicode.org/
[2] https://github.com/quantumlib/OpenFermion