Singlet–Triplet Energy Gaps of Organic Biradicals
and Polyacenes with Auxiliary-Field Quantum Monte Carlo
Posted on 2019-08-19 - 18:44
The energy gap between the lowest-lying
singlet and triplet states
is an important quantity in chemical photocatalysis, with relevant
applications ranging from triplet fusion in optical upconversion to
the design of organic light-emitting devices. The ab initio prediction of singlet–triplet (ST) gaps is challenging due
to the potentially biradical nature of the involved states, combined
with the potentially large size of relevant molecules. In this work,
we show that phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC)
can accurately predict ST gaps for chemical systems with singlet states
of highly biradical nature, including a set of 13 small molecules
and the ortho-, meta-, and para- isomers of benzyne. With respect to gas-phase experiments,
ph-AFQMC using CASSCF trial wave functions achieves a mean averaged
error of ∼1 kcal/mol. Furthermore, we find that in the context
of a spin-projection technique, ph-AFQMC using unrestricted single-determinant
trial wave functions, which can be readily obtained for even very
large systems, produces equivalently high accuracy. We proceed to
show that this scalable methodology is capable of yielding accurate
ST gaps for all linear polyacenes for which experimental measurements
exist, that is, naphthalene, anthracene, tetracene, and pentacene.
Our results suggest a protocol for selecting either unrestricted Hartree–Fock
or Kohn–Sham orbitals for the single-determinant trial wave
function, based on the extent of spin-contamination. These findings
pave the way for future investigations of specific photochemical processes
involving large molecules with substantial biradical character.
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Shee, James; Arthur, Evan J.; Zhang, Shiwei; Reichman, David R.; Friesner, Richard A. (2019). Singlet–Triplet Energy Gaps of Organic Biradicals
and Polyacenes with Auxiliary-Field Quantum Monte Carlo. ACS Publications. Collection. https://doi.org/10.1021/acs.jctc.9b00534