3,851 research outputs found
Analytical derivative coupling for multistate CASPT2 theory
The probability of non-radiative transitions in photochemical dynamics is
determined by the derivative couplings, the couplings between different
electronic states through the nuclear degrees of freedom. Efficient and
accurate evaluation of the derivative couplings is, therefore, of central
importance to realize reliable computer simulations of photochemical reactions.
In this work, the derivative couplings for multistate multireference
second-order perturbation theory (MS-CASPT2) and its 'extended' variant
(XMS-CASPT2) are studied, in which we present an algorithm for their analytical
evaluation. The computational costs for evaluating the derivative couplings are
essentially the same as those for calculating the nuclear energy gradients. The
geometries and energies calculated with XMS-CASPT2 for small molecules at
minimum energy conical intersections (MECIs) are in good agreement with those
computed by multireference configuration interaction. As numerical examples,
MECIs are optimized using XMS-CASPT2 for stilbene and a GFP model chromophore
(the 4-para-hydroxybenzylidene-1,2-dimethyl-imidazolin-5-one anion)
Assessing Excited State Energy Gaps with Time-Dependent Density Functional Theory on Ru(II) Complexes
A set of density functionals coming from different rungs on Jacob's ladder
are employed to evaluate the electronic excited states of three Ru(II)
complexes. While most studies on the performance of density functionals compare
the vertical excitation energies, in this work we focus on the energy gaps
between the electronic excited states, of the same and different multiplicity.
Excited state energy gaps are important for example to determine radiationless
transition probabilities. Besides energies, a functional should deliver the
correct state character and state ordering. Therefore, wavefunction overlaps
are introduced to systematically evaluate the effect of different functionals
on the character of the excited states. As a reference, the energies and state
characters from multi-state second-order perturbation theory complete active
space (MS-CASPT2) are used. In comparison to MS-CASPT2, it is found that while
hybrid functionals provide better vertical excitation energies, pure
functionals typically give more accurate excited state energy gaps. Pure
functionals are also found to reproduce the state character and ordering in
closer agreement to MS-CASPT2 than the hybrid functionals
On-the-fly CASPT2 surface hopping dynamics
We report the development of programs for on-the-fly surface hopping dynamics
simulations in the gas and condensed phases on the potential energy surfaces
computed by multistate multireference perturbation theory (XMS-CASPT2) with
full internal contraction. On-the-fly nonadiabatic dynamics simulations are
made possible by improving the algorithm for XMS-CASPT2 nuclear energy gradient
and derivative coupling evaluation. The program is interfaced to a surface
hopping dynamics program, Newton-X, and a classical molecular dynamics package,
tinker, to realize such simulations. On-the-fly XMS-CASPT2 surface-hopping
dynamics simulations of 9H-adenine and an anionic GFP model chromophore
(para-hydroxybenzilideneimidazolin-5-one) in water are presented to demonstrate
the applicability of our program to sizable systems. Our program is implemented
in the bagel package, which is publicly available under the GNU General Public
License
On the accuracy of retinal protonated Schiff base models
We investigate the molecular geometries of the ground state and the minimal
energy conical intersections (MECIs) between the ground and first excited
states of the models for the retinal protonated Schiff base in the gas phase
using the extended multistate complete active space second-order perturbation
theory (XMS-CASPT2). The biggest model in this work is the rhodopsin
chromophore truncated between the {\epsilon} and {\delta} carbon atoms, which
consists of 54 atoms and 12-orbital {\pi} conjugation. The results are compared
with those obtained by the state-averaged complete active space self-consistent
field (SA-CASSCF). The XMS-CASPT2 results suggest that the minimum energy
conical intersection associated with the so-called 13-14 isomerization is
thermally inaccessible, which is in contrast to the SA-CASSCF results. The
differences between the geometries of the conical intersections computed by
SA-CASSCF and XMS-CASPT2 are ascribed to the fact that the charge transfer
states are more stabilized by dynamical electron correlation than the
diradicaloid states. The impact of the various choices of active spaces, basis
sets, and state averaging schemes is also examined.Comment: Contribution to the special issue in honor of the 80th birthday of
Professor Michael Bae
Excited State Specific Multi-Slater Jastrow Wave Functions
We combine recent advances in excited state variational principles, fast
multi-Slater Jastrow methods, and selective configuration interaction to create
multi-Slater Jastrow wave function approximations that are optimized for
individual excited states. In addition to the Jastrow variables and linear
expansion coefficients, this optimization includes state-specific orbital
relaxations in order to avoid the compromises necessary in state-averaged
approaches. We demonstrate that, when combined with variance matching to help
balance the quality of the approximation across different states, this approach
delivers accurate excitation energies even when using very modest multi-Slater
expansions. Intriguingly, this accuracy is maintained even when studying a
difficult chlorine-anion-to- charge transfer in which traditional
state-averaged multi-reference methods must contend with different states that
require drastically different orbital relaxations.Comment: 16 pages, 6 figures, 2 table
A comparative analysis of the UV/Vis absorption spectra of nitrobenzaldehydes.
In a joint experimental and theoretical study, the UV/Vis absorption spectra of the three isomers (ortho, meta, para) of nitrobenzaldehyde (NBA) were analyzed. Absorption spectra are reported for NBA vapors, cyclohexane and acetonitrile solutions. All spectra are poor in vibronic structure and hardly affected in shape by the surroundings (vapor or solution). Moderate solvatochromic shifts of [similar]−0.2 eV are measured. For all isomers vertical transition energies, oscillator strengths, and excited state dipole moments were computed using the MS-CASPT2/CASSCF and CC2 methods. Based on these calculations the experimental transitions were assigned. The spectra of all isomers are characterized by weak (εmax ≈ 100 M−1 cm−1) transitions around 350 nm (3.6 eV), arising from nπ* absorptions starting from the lone pairs of the nitro and aldehyde moieties. The next band of intermediate intensity peaking around 300 nm (4.2 eV, εmax ≈ 1000 M−1 cm−1) is dominated by ππ* excitations within the arene function. Finally, strong absorptions (εmax ≈ 10 000 M−1 cm−1) were observed around 250 nm (5.0 eV) which we ascribe to ππ* excitations involving the nitro and benzene groups
Exploration of H2 binding to the [NiFe]-hydrogenase active site with multiconfigurational density functional theory
The combination of density functional theory (DFT) with a
multiconfigurational wave function is an efficient way to include dynamical
correlation in calculations with multiconfiguration self-consistent field wave
functions. These methods can potentially be employed to elucidate reaction
mechanisms in bio-inorganic chemistry, where many other methods become either
too computationally expensive or too inaccurate. In this paper, a complete
active space (CAS) short-range DFT (CAS-srDFT) hybrid was employed to
investigate a bio-inorganic system, namely H2 binding to the active site of
[NiFe] hydrogenase. This system was previously investigated with
coupled-cluster (CC) and multiconfigurational methods in form of
cumulant-approximated second-order perturbation theory, based on the density
matrix renormalization group (DMRG). We find that it is more favorable for H2
to bind to Ni than to Fe, in agreement with previous CC and DMRG calculations.
The accuracy of CAS-srDFT is comparable to both CC and DMRG, despite that much
smaller active spaces were employed. This enhanced efficiency at smaller active
spaces shows that CAS-srDFT can become a useful method for bio-inorganic
chemistry.Comment: 22 page
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