Quantum Monte Carlo for the x-ray absorption spectrum of pyrrole at the nitrogen K-edge

Abstract

Fixed-node diffusion Monte Carlo (FNDMC) is used to simulate the x-ray absorption spectrum of a gas-phase pyrrole molecule at the nitrogen K-edge. Trial wave functions for core-excited states are constructed from ground-state Kohn-Sham determinants substituted with singly occupied natural orbitals from configuration interaction with single excitations calculations of the five lowest valence-excited triplet states. The FNDMC ionization potential (IP) is found to lie within 0.3 eV of the experimental value of 406.1 ± 0.1 eV. The transition energies to anti-bonding virtual orbitals match the experimental spectrum after alignment of IP values and agree with the existing assignments.

A promising intersection of excited‐state‐specific methods from quantum chemistry and quantum Monte Carlo

“…Although most uses of QMC methods focus on the ground state, there has also been a substantial amount of work applying QMC, and especially VMC, to excited states. In many cases, and especially in earlier applications, researchers have relied on quantum chemistry methods like DFT and CASSCF to determine the fermionic part of the wave function, with only the Jastrow factor optimized within VMC 130–135 . Although in this Focus Article we will for efficiency's sake advocate a partial return to this reliance on quantum chemistry in the context of excited‐state‐specific CASSCF, we emphasize that these early uses of quantum chemistry to determine the final orbitals and/or determinant coefficients typically did not have the luxury of employing excited‐state‐specific quantum chemistry methods.…”