Static and dynamical correlation in diradical molecules by Quantum Monte Carlo using the Jastrow Antisymmetrized Geminal Power ansatz

TL;DR

This paper demonstrates that the Jastrow Antisymmetrized Geminal Power (JAGP) wave function within Quantum Monte Carlo effectively captures static and dynamical correlation in diradical molecules, outperforming simpler models.

Contribution

The study shows that JAGP accurately describes diradical electronic structures with computational efficiency comparable to single determinant methods.

Findings

JAGP provides a reliable description of diradicals like ethylene and methylene.

Single determinant Jastrow wave functions fail to capture static correlation.

JAGP's scalability enables studies of large diradical systems.

Abstract

Diradical molecules are essential species involved in many organic and inorganic chemical reactions. The computational study of their electronic structure is often challenging, because a reliable description of the correlation, and in particular of the static one, requires multi-reference techniques. The Jastrow correlated Antisymmetrized Geminal Power (JAGP) is a compact and efficient wave function ansatz, based on the valence-bond representation, which can be used within Quantum Monte Carlo (QMC) approaches. The AGP part can be rewritten in terms of molecular orbitals, obtaining a multi-determinant expansion with zero-seniority number. In the present work we demonstrate the capability of the JAGP ansatz to correctly describe the electronic structure of two diradical prototypes: the orthogonally twisted ethylene, C2H4, and the methylene, CH2, representing respectively a homosymmetric and heterosymmetric system. On the other hand, we show that the simple ansatz of a Jastrow correlated Single Determinant (JSD) wave function is unable to provide an accurate description of the electronic structure in these diradical molecules, both at variational level and, more remarkably, in the fixed-nodes projection schemes showing that a poor description of the static correlation yields an inaccurate nodal surface. The suitability of JAGP to correctly describe diradicals with a computational cost comparable with that of a JSD calculation, in combination with a favorable scalability of QMC algorithms with the system size, opens new perspectives in the ab initio study of large diradical systems, like the transition states in cycloaddition reactions and the thermal isomerization of biological chromophores.