TY - JOUR

T1 - Influence of triplet instabilities in TDDFT

AU - Peach, Michael J. G.

AU - Williamson, Matthew J.

AU - Tozer, David J.

PY - 2011/11

Y1 - 2011/11

N2 - Singlet and triplet vertical excitation energies from time-dependent density functional theory (TDDFT) can be affected in different ways by the inclusion of exact exchange in hybrid or Coulomb-attenuated/range-separated exchange-correlation functionals; in particular, triplet excitation energies can become significantly too low. To investigate these issues, the explicit dependence of excitation energies on exact exchange is quantified for four representative molecules, paying attention to the effect of constant, short-range, and long-range contributions. A stability analysis is used to verify that the problematic TDDFT triplet excitations can be understood in terms of the ground state triplet instability problem, and it is proposed that a Hartree-Fock stability analysis should be used to identify triplet excitations for which the presence of exact exchange in the TDDFT functional is undesirable. The use of the Tamm-Dancoff approximation (TDA) significantly improves the problematic triplet excitation energies, recovering the correct state ordering in benzoquinone; it also affects the corresponding singlet states, recovering the correct state ordering in naphthalene. The impressive performance of the TDA is maintained for a wide range of molecules across representative functionals.

AB - Singlet and triplet vertical excitation energies from time-dependent density functional theory (TDDFT) can be affected in different ways by the inclusion of exact exchange in hybrid or Coulomb-attenuated/range-separated exchange-correlation functionals; in particular, triplet excitation energies can become significantly too low. To investigate these issues, the explicit dependence of excitation energies on exact exchange is quantified for four representative molecules, paying attention to the effect of constant, short-range, and long-range contributions. A stability analysis is used to verify that the problematic TDDFT triplet excitations can be understood in terms of the ground state triplet instability problem, and it is proposed that a Hartree-Fock stability analysis should be used to identify triplet excitations for which the presence of exact exchange in the TDDFT functional is undesirable. The use of the Tamm-Dancoff approximation (TDA) significantly improves the problematic triplet excitation energies, recovering the correct state ordering in benzoquinone; it also affects the corresponding singlet states, recovering the correct state ordering in naphthalene. The impressive performance of the TDA is maintained for a wide range of molecules across representative functionals.

KW - EXCHANGE

KW - GENERALIZED-GRADIENT-APPROXIMATION

KW - DENSITY-FUNCTIONAL THEORY

KW - CONFIGURATION-INTERACTION

KW - EXCITED-STATES

KW - MOLECULES

KW - TAMM-DANCOFF APPROXIMATION

KW - HARTREE-FOCK

KW - ABSORPTION

KW - EXCITATION-ENERGIES

UR - http://www.scopus.com/inward/record.url?scp=80755155855&partnerID=8YFLogxK

U2 - 10.1021/ct200651r

DO - 10.1021/ct200651r

M3 - Journal article

VL - 7

SP - 3578

EP - 3585

JO - Journal of Chemical Theory and Computation

JF - Journal of Chemical Theory and Computation

SN - 1549-9618

IS - 11

ER -