Figure 5.

Perturbative energy calculation on the butadiene molecule with a FCI space of (22e,62o) using the cc-pVDZ basis for hydrogen and SV basis for carbon. The RHF configuration is chosen as the |0⟩th order wavefunction |Ψ₀⟩. a.) The PT2 energy density defined as the number of MPBFs with a given magnitude of E_I^PT2 energy contribution (in millihartree). The distribution is close to an asymmetric Gaussian with long tails, as expected. The individual contribution of each MPBF is given in panel (b). The total number of singles and doubles is larger for DET MPBF than CFG and CSF MPBFs, as expected. The dashed lines indicate the peak values of the PT2 contributions for each curve to enable a quantitative comparison.