3 research outputs found
Multireference approaches for excited states of molecules
Understanding the properties of electronically excited states is a challenging task that becomes increasingly important for numerous applications in chemistry, molecular physics, molecular biology, and materials science. A substantial impact is exerted by the fascinating progress in time-resolved spectroscopy, which leads to a strongly growing demand for theoretical methods to describe the characteristic features of excited states accurately. Whereas for electronic ground state problems of stable molecules the quantum chemical methodology is now so well developed that informed nonexperts can use it efficiently, the situation is entirely different concerning the investigation of excited states. This review is devoted to a specific class of approaches, usually denoted as multireference (MR) methods, the generality of which is needed for solving many spectroscopic or photodynamical problems. However, the understanding and proper application of these MR methods is often found to be difficult due to their complexity and their computational cost. The purpose of this review is to provide an overview of the most important facts about the different theoretical approaches available and to present by means of a collection of characteristic examples useful information, which can guide the reader in performing their own applications
Singlet L<sub>a</sub> and L<sub>b</sub> Bands for N‑Acenes (<i>N</i> = 2–7): A CASSCF/CASPT2 Study
In
this work CASPT2 calculations of polyacenes (from naphthalene
to heptacene) were performed to find a methodology suitable for calculations
of the absorption spectra, in particular of the L<sub>a</sub> (B<sub>2u</sub> state) and L<sub>b</sub> (B<sub>3u</sub> state) bands, of
more extended systems. The effect of the extension of the active space
and of freezing σ orbitals was investigated. The MCSCF excitation
energy of the B<sub>2u</sub> state is not sensitive to the size of
the active space used. However, the CASPT2 results depend strongly
on the amount of σ orbitals frozen reflecting the ionic character
of the B<sub>2u</sub> state. On the other hand, the excitation energies
of the B<sub>3u</sub> state are much more sensitive to the size of
the active space used in the calculations reflecting its multiconfigurational
character. We found a good agreement with experimental data for both
bands by including 14 electrons in 14 π orbitals in the active
space followed by the CASPT2(14,14) perturbation scheme in which both
σ and π orbitals are included
Strikingly Different Effects of Hydrogen Bonding on the Photodynamics of Individual Nucleobases in DNA: Comparison of Guanine and Cytosine
Ab initio surface hopping dynamics calculations were
performed
to study the photophysical behavior of cytosine and guanine embedded
in DNA using a quantum mechanical/molecular mechanics (QM/MM) approach.
It was found that the decay rates of photo excited cytosine and guanine
were affected in a completely different way by the hydrogen bonding
to the DNA environment. In case of cytosine, the geometrical restrictions
exerted by the hydrogen bonds did not influence the relaxation time
of cytosine significantly due to the generally small cytosine ring
puckering required to access the crossing region between excited and
ground state. On the contrary, the presence of hydrogen bonds significantly
altered the photodynamics of guanine. The analysis of the dynamics
indicates that the major contribution to the lifetime changes comes
from the interstrand hydrogen bonds. These bonds considerably restricted
the out-of-plane motions of the NH<sub>2</sub> group of guanine which
are necessary for the ultrafast decay to the ground state. As a result,
only a negligible amount of trajectories decayed into the ground state
for guanine embedded in DNA within the simulation time of 0.5 ps,
while for comparison, the isolated guanine relaxed to the ground state
with a lifetime of about 0.22 ps. These examples show that, in addition
to phenomena related to electronic interactions between nucleobases,
there also exist relatively simple mechanisms in DNA by which the
lifetime of a nucleobase is significantly enhanced as compared to
the gas phase