Communication: Orbital instabilities and triplet states from time-dependent density functional theory and long-range corrected functionals

John S Sears,Jean-Luc Brédas,Cai-Rong Zhang,Thomas Koerzdoerfer

doi:10.1063/1.3656734

John S Sears, Jean-Luc Brédas + Show 2 more

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https://doi.org/10.1063/1.3656734

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Abstract

Long-range corrected hybrids represent an increasingly popular class of functionals for density functional theory (DFT) that have proven to be very successful for a wide range of chemical applications. In this Communication, we examine the performance of these functionals for time-dependent (TD)DFT descriptions of triplet excited states. Our results reveal that the triplet energies are particularly sensitive to the range-separation parameter; this sensitivity can be traced back to triplet instabilities in the ground state coming from the large effective amounts of Hartree-Fock exchange included in these functionals. As such, the use of standard long-range corrected functionals for the description of triplet states at the TDDFT level is not recommended.

Highlights

IntroductionDensity functional theory (DFT) and its time-dependent extension (time-dependent density functional theory; TDDFT) have become the methods of choice for quantummechanical applications in many areas of chemistry.Recently, long-range corrected (LRC) hybrid functionals have generated a significant amount of attention in the literature. they have been shown to improve upon the standard hybrid functionals for numerous properties of particular interest; examples include: fundamental gaps and ionization potentials (IPs), bond-length alternations in π -conjugated materials, molecular polarizabilities and hyperpolarizabilities, or vibrational frequencies and IR/Raman intensities. Primarily, it is the outstanding performance of LRC-hybrids for charge-transfer excitations that makes this new class of functionals interesting for TDDFT applications in organic electronics.7–9The central premise underlying all LRC functionals is a separation of the Coulomb operator into short-range (SR) and long-range (LR) components that can be treated separately.For instance, a semilocal exchange-correlation functional can be used for the SR and Hartree-Fock for LR
Long-range corrected (LRC) hybrid functionals have generated a significant amount of attention in the literature
The presence of these instabilities has a significant impact on the description of triplet states with these functionals. In this Communication, we explore the nature of the orbital instabilities and the TDDFT description of the triplet states in linear acenes for several long-range corrected (LRC) functionals

Summary

Introduction

Density functional theory (DFT) and its time-dependent extension (time-dependent density functional theory; TDDFT) have become the methods of choice for quantummechanical applications in many areas of chemistry.Recently, long-range corrected (LRC) hybrid functionals have generated a significant amount of attention in the literature. they have been shown to improve upon the standard hybrid functionals for numerous properties of particular interest; examples include: fundamental gaps and ionization potentials (IPs), bond-length alternations in π -conjugated materials, molecular polarizabilities and hyperpolarizabilities, or vibrational frequencies and IR/Raman intensities. Primarily, it is the outstanding performance of LRC-hybrids for charge-transfer excitations that makes this new class of functionals interesting for TDDFT applications in organic electronics.7–9The central premise underlying all LRC functionals is a separation of the Coulomb operator into short-range (SR) and long-range (LR) components that can be treated separately.For instance, a semilocal exchange-correlation functional can be used for the SR and Hartree-Fock for LR. Long-range corrected (LRC) hybrid functionals have generated a significant amount of attention in the literature.1 They have been shown to improve upon the standard hybrid functionals for numerous properties of particular interest; examples include: fundamental gaps and ionization potentials (IPs), bond-length alternations in π -conjugated materials, molecular polarizabilities and hyperpolarizabilities, or vibrational frequencies and IR/Raman intensities.. They have been shown to improve upon the standard hybrid functionals for numerous properties of particular interest; examples include: fundamental gaps and ionization potentials (IPs), bond-length alternations in π -conjugated materials, molecular polarizabilities and hyperpolarizabilities, or vibrational frequencies and IR/Raman intensities.6 It is the outstanding performance of LRC-hybrids for charge-transfer excitations that makes this new class of functionals interesting for TDDFT applications in organic electronics.. The most popular approach to the range separation (and the one employed in this work) is to partition the Coulomb operator via the standard error function:

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