Paul A Johnson
@johnsontheochem.bsky.social
Strong electron correlation with Bethe Ansatz wavefunctions
CASSCF quality dissociations with MP2 cost. Single-reference perturbation theory with Perfect-Pairing reference. The excitations are correlated but everything is simple to compute. arxiv.org/abs/2605.31582
Richardson-Gaudin states of non-zero seniority III: The Perfect-Pairing limit
Strongly correlated electrons can be treated with a configuration interaction of Slater determinants grouped by number of unpaired electrons with exponential cost. The first two papers in this series ...
arxiv.org
Université Laval: Canada Impact+ Research Chairs in Nanostructured Materials Chemistry https://chempostingscanada.blogspot.com/2026/02/universite-laval-canada-impact-research.html
Work with @titouloos.bsky.social @marm314.bsky.social : PP is an independent pair model with a reference Hamiltonian. The corresponding PT2 correction \approx pCCD. Here, seniority-zero correction only in the valence. The real correction coming soon.
Connections between Richardson–Gaudin states, perfect-pairing, and pair coupled-cluster theory
Slater determinants underpin most electronic structure methods, but orbital-based approaches often struggle to describe strong correlation efficiently. Geminal-
pubs.aip.org
arxiv.org/abs/2510.06144 #compchem
Connections between Richardson-Gaudin States, Perfect-Pairing, and Pair Coupled-Cluster Theory
Slater determinants underpin most electronic structure methods, but orbital-based approaches often struggle to describe strong correlation efficiently. Geminal-based theories, by contrast, naturally c...
arxiv.org
arxiv.org/abs/2507.02160 #compchem
Fully Analytic Nuclear Gradients for the Bethe--Salpeter Equation
The Bethe-Salpeter equation (BSE) formalism, combined with the $GW$ approximation for ionization energies and electron affinities, is emerging as an efficient and accurate method for predicting optica...
arxiv.org
Very short follow-up after the ridiculous part I. Short excitation-based CI of RG states converges quickly. (ordinarily, I'd reply to a post from @titouloos.bsky.social ) arxiv.org/abs/2506.09379
Richardson-Gaudin states of non-zero seniority II: Single-reference treatment of strong correlation
Strongly correlated systems are well described as a configuration interaction of Slater determinants classified by their number of unpaired electrons. This treatment is however unfeasible. In this man...
arxiv.org
Finally. This was quite a struggle. In short: can add missing structure from a seniority-zero state with single-reference methods. Parts 2 and 3 to come. pubs.aip.org/aip/jcp/arti...
Richardson–Gaudin states of non-zero seniority: Matrix elements
Seniority-zero wave functions describe bond-breaking processes qualitatively. As eigenvectors of a model Hamiltonian, Richardson–Gaudin states provide a clear p
pubs.aip.org
Scheduling a meeting at 6:30, google calendar assumes it's in the evening 😂
Seniority-based CI works well for strongly correlated systems at an exponential cost. The goal here is to replace it with a short excitation-based CI of correlated states. Here all the required matrix elements are computed. In every case, a single linear algebra operation per pair of states.
Pierre-Francois Loos@titouloos.bsky.social · 2y ago