Method For Making 2-Electron Response Reduced Density Matrices Approximately N-representable

Understand

In methods like geminal-based approaches or coupled cluster that are solved using the projected Schr\"odinger equation, direct computation of the 2-electron reduced density matrix (2-RDM) is impractical and one falls back to a 2-RDM based on response theory.

• However, the 2-RDMs from response theory are not $N$-representable.
• That is, the response 2-RDM does not correspond to an actual physical $N$-electron wave function.
• We present a new algorithm for making these non-$N$-representable 2-RDMs approximately $N$-representable, i.e.