22 research outputs found
Conical Intersection and Potential Energy Surface Features of a Model Retinal Chromophore: Comparison of EOM-CC and Multireference Methods
This work investigates the performance of equation-of-motion
coupled-cluster
(EOM-CC) methods for describing the changes in the potential energy
surfaces of the penta-2,4-dieniminium cation, a reduced model of the
retinal chromophore of visual pigments, due to dynamical electron
correlation effects. The ground-state wave function of this model
includes charge-transfer and diradical configurations whose weights
vary along different displacements and are rapidly changing at the
conical intersection between the ground and the first excited states,
making the shape of the potential energy surface sensitive to a balanced
description of nondynamical and dynamical correlation. Recently, variational
(MRCISD) and perturbative (MRPT2) approaches for including dynamical
correlation in CASSCF-based calculations were tested along three representative
ground state paths. Here, we use the same three paths to compare the
performance of single-reference EOM-CC methods against MRCISD and
MRCISD+Q. We find that the spin-flip variant of EOM-CCSD with perturbative
inclusion of triple excitations (dT or fT) produces potential energy
profiles of the two lowest electronic states in quantitative agreement
with MRCISD+Q (our highest-quality reference method). The nonparallelity
errors and differences in vertical energy differences of the two surfaces
along these scans are less than 1.4 kcal/mol (EOM-SF-CCSDĀ(dT) versus
MRCISD+Q). For comparison, the largest error of MRCISD versus MRCISD+Q
is 1.7 kcal/mol. Our results show that the EOM-CC methods provide
an alternative to multireference approaches and may be used to study
photochemical systems like the one used in this work
Combined Self-Consistent-Field and Spin-Flip TammāDancoff Density Functional Approach to Potential Energy Surfaces for Photochemistry
We present a new approach to calculating potential energy
surfaces
for photochemical reactions by combining self-consistent-field calculations
for single-reference ground and excited states with symmetry-corrected
spin-flip TammāDancoff approximation calculations for multireference
electronic states. The method is illustrated by an application with
the M05-2X exchange-correlation functional to <i>cis</i>ā<i>trans</i> isomerization of the penta-2,4-dieniminium
cation, which is a model (with three conjugated double bonds) of the
protonated Schiff base of retinal. We find good agreement with multireference
configuration interaction-plus-quadruples (MRCISD+Q) wave function
calculations along three key paths in the strong-interaction region
of the ground and first excited singlet states
Assessment of Density Functional Theory for Describing the Correlation Effects on the Ground and Excited State Potential Energy Surfaces of a Retinal Chromophore Model
In
the quest for a cost-effective level of theory able to describe
a large portion of the ground and excited potential energy surfaces
of large chromophores, promising approaches are rooted in various
approximations to the exact density functional theory (DFT). In the
present work, we investigate how generalized KohnāSham DFT
(GKS-DFT), time-dependent DFT (TDDFT), and spin-restricted ensemble-DFT
(REKS) methods perform along three important paths characterizing
a model retinal chromophore (the penta-2,4-dieniminium cation) in
a region of near-degeneracy (close to a conical intersection) with
respect to reference high-level multiconfigurational wave function
methods. If GKS-DFT correctly describes the closed-shell charge transfer
state, only TDDFT and REKS approaches give access to the open-shell
diradical, one which sometimes corresponds to the electronic ground
state. It is demonstrated that the main drawback of the usual DFT-based
methods lies in the absence of interactions between the charge transfer
and the diradicaloid configurations. Hence, we test a new computational
scheme based on the State-averaged REKS (SA-REKS) approach, which
explicitly includes these interactions into account. The State-Interaction
SA-REKS (SI-SA-REKS) method significantly improves on the REKS and
the SA-REKS results for the target system. The similarities and differences
between DFT and wave function-based approaches are analyzed according
to (1) the active space dimensions of the wave function-based methods
and (2) the relative electronegativities of the allyl and protonated
Schiff base moieties
Electronic Spectra of Tris(2,2ā²-bipyridine)-M(II) Complex Ions in Vacuo (M = Fe and Os)
We measured the electronic spectra
of mass-selected [MĀ(bpy)<sub>3</sub>]<sup>2+</sup> (M = Fe and Os,
bpy = 2,2ā²-bipyridine) ions in vacuo by photodissociation spectroscopy
of their N<sub>2</sub> adducts, [MĀ(bpy)<sub>3</sub>]<sup>2+</sup>Ā·N<sub>2</sub>. Extensive band systems in the visible (predominantly charge
transfer) and near-ultraviolet (ĻĻ*) spectral regions
are reported. The [MĀ(bpy)<sub>3</sub>]<sup>2+</sup>Ā·N<sub>2</sub> target ions were prepared by condensing N<sub>2</sub> onto electrosprayed
ions in a cryogenic ion trap at ca. 25 K and then mass-selected by
time-of-flight mass spectrometry. The electronic photodissociation
spectra of the cold, gas-phase ions closely reflect their intrinsic
properties, i.e., without perturbation by solvent effects. The spectra
are interpreted using time-dependent density functional theory calculations
both with and without accounting for relativistic effects
Origin of Fluorescence in 11-<i>cis</i> Locked Bovine Rhodopsin
The excited state lifetime of bovine rhodopsin (Rh) increases
from
ca. 100 fs to 85 ps when the C11ī»C12 bond of its chromophore
is locked by a cyclopentene moiety (Rh5). To explain such an increase,
we employed <i>ab initio</i> multiconfigurational quantum
chemistry to construct computer models of Rh and Rh5 and to investigate
the shape of their excited state potential energy surfaces in a comparative
way. Our results show that the observed Rh5 fluorescence (Ī»<sub>max</sub><sup>f</sup> = 620 nm) is due to a previously unreported
locally excited intermediate whose lifetime is controlled by a small
energy barrier. The analysis of the properties and decay path of such
an intermediate provides useful information for engineering rhodopsin
variants with augmented fluorescence efficiencies
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources
Probing Vibrationally Mediated Ultrafast Excited-State Reaction Dynamics with Multireference (CASPT2) Trajectories
Excited-state trajectories computed
at the complete active space
second-order perturbation theory (CASPT2) reveal how vibrational excitation
controls the molecular approach to the intersection space that drives
the photodissociation of a prototypical halogenated methyl radical,
namely CF<sub>2</sub>I. Translating the FranckāCondon structure
along the ground-state CASPT2 vibrational modes in this system followed
by propagating the displaced structures in the first excited doublet
state simulates specific vibrational excitations and vibrationally
mediated dynamics, respectively. Three distinct situations are encountered:
the trajectories (i) converge to an energetically flat segment of
the intersection space, (ii) locate a segment of the intersection
space, and (iii) access a region where the intersection space degeneracy
is lifted to form a ridge of avoided crossings. The computational
protocol documented herein can be used as a tool to design control
strategies based on selective excitation of vibrational modes, including
adaptive feedback schemes using coherent light sources