Multiconfiguration Pair-Density Functional Theory Calculations of Iron(II) Porphyrin: Effects of Hybrid Pair-Density Functionals and Expanded RAS and DMRG Active Spaces on Spin-State Orderings

Gautam D. Stroscio, Chen Zhou, Donald G. Truhlar, Laura Gagliardi

Research output: Contribution to journalArticlepeer-review

11 Scopus citations

Abstract

Iron(II) porphyrins play critical roles in enzymes and synthetic catalysts. Computationally determining the spin-state ordering for even the unsubstituted iron(II) porphyrin (FeP) is challenging due to its large size. Multiconfiguration pair-density functional theory (MC-PDFT), a method capable of accurately capturing correlation with lower cost than comparably accurate methods, was previously used to predict a triplet ground state for FeP across a wide range of active spaces up to (34e, 35o). The purpose of this present MC-PDFT study is to determine the effects of including nonlocal exchange in the energy calculation and of using a larger active space size [DMRG(40e, 42o) and RAS(40, 2, 2; 16, 6, 20)] on the calculated FeP spin-state ordering. The recently developed hybrid MC-PDFT method, which uses a weighted average of the MC-PDFT energy and the energy expectation value of the reference wave function, is applied with a weight of the reference wave function energy of λ. We find that increasing λ stabilizes the quintet relative to the triplets. The hybrid tPBE0 functional (tPBE with λ set to 0.25) consistently predicts a triplet ground state with the quintet lying above by 0.10-0.16 eV, depending on the reference wave function. These values are particularly interesting in light of tPBE0's very strong performance for a diverse set of other systems.

Original languageEnglish (US)
Pages (from-to)3957-3963
Number of pages7
JournalJournal of Physical Chemistry A
Volume126
Issue number24
DOIs
StatePublished - Jun 23 2022

Bibliographical note

Funding Information:
The authors thank Daniel King and Dr. Stefan Knecht for helpful discussions and email correspondence related to using DMRG in the QCMaquis software. The authors also gratefully acknowledge the University of Chicago Research Computing Center (RCC), which provided the computational resources for this project. The present work was supported in part by the Air Force Office of Scientific Research by Grant FA9550-20-1-0360.

Publisher Copyright:
© 2022 American Chemical Society. All rights reserved.

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