10 research outputs found
Femtosecond intramolecular rearrangement of the CH3NCS radical cation
Strong-field ionization, involving tunnel ionization and electron
rescattering, enables femtosecond time-resolved dynamics measurements of
chemical reactions involving radical cations. Here, we compare the formation of
CH3S+ following the strong-field ionization of the isomers CH3SCN and CH3NCS.
The former involves the release of neutral CN, while the latter involves an
intramolecular rearrangement. We find the intramolecular rearrangement takes
place on the single picosecond timescale and exhibits vibrational coherence.
Density functional theory and coupled-cluster calculations on the neutral and
singly ionized species help us determine the driving force responsible for
intramolecular rearrangement in CH3NCS. Our findings illustrate the complexity
that accompanies radical cation chemistry following electron ionization and
demonstrate a useful tool for understanding the cation dynamics after
ionization.Comment: Combined PDF file consisting of the main text (20 pages, 7 figures, 2
tables) and the supplementary material (7 pages, 1 figure, nuclear
coordinates of the calculated molecular structures). This article has been
accepted for publication in the Journal of Chemical Physics. After it is
published, it will be found at https://doi.org/10.1063/5.011787
N-Representability Violations in Truncated Equation-of-Motion Coupled-Cluster Methods
One-electron reduced density matrices (1RDMs) from equation-of-motion (EOM)
coupled-cluster with single and double excitations (CCSD) calculations are
analyzed to assess their N-representability (i.e., whether they are derivable
from an physical N-electron state). We identify EOM-CCSD stationary states
whose 1RDMs violate either ensemble-state N-representability conditions or
pure-state conditions known as generalized Pauli constraints (GPCs). As such,
these 1RDMs do not correspond to any physical N-electron wave function.
Unphysical states are also encountered in the course of time-dependent EOM-CC
simulations; when an external field drives transitions between a pair of
stationary states with pure-state N-representable 1RDMs, the 1RDM of the
time-dependent state can violate ensemble-state conditions. These observations
point to potential challenges in interpreting the results of time-dependent
EOM-CCSD simulations
Benchmarking the Semi-Stochastic CC(P;Q) Approach for Singlet-Triplet Gaps in Biradicals
We recently developed a semi-stochastic approach to converging high-level
coupled-cluster (CC) energetics, such as those obtained in the CC calculations
with singles, doubles, and triples (CCSDT), in which the deterministic
CC(;) framework is merged with the stochastic configuration interaction
Quantum Monte Carlo (CIQMC) propagations [J. E. Deustua, J. Shen, and P.
Piecuch, Phys. Rev. Lett. 119, 223003 (2017)]. In this work, we investigate the
ability of the semi-stochastic CC(;) methodology to recover the CCSDT
energies of the lowest singlet and triplet states and the corresponding
singlet-triplet gaps of biradical systems using methylene, ,
cyclobutadiene, cyclopentadienyl cation, and trimethylenemethane as
representative examples. We demonstrate that the semi-stochastic CC(;)
calculations are capable of generating results of the CCSDT quality and
improving the singlet-triplet gaps obtained with the triples corrections to
CCSD defining the CR-CC(2,3) approach out of the early stages of CIQMC
propagations.Comment: 31 pages, 12 tables, 5 figures. This article has been submitted to
the Journal of Chemical Physic
Time-Dependent Equation-of-Motion Coupled-Cluster Simulations with a Defective Hamiltonian
Simulations of laser-induced electron dynamics in a molecular system are
performed using time-dependent (TD) equation-of-motion (EOM) coupled-cluster
(CC) theory. The target system has been chosen to highlight potential
shortcomings of truncated TD-EOM-CC methods [represented in this work by
TD-EOM-CC with single and double excitations (TD-EOM-CCSD)], where unphysical
spectroscopic features can emerge. Specifically, we explore driven resonant
electronic excitations in magnesium fluoride in the proximity of an avoided
crossing. Near the avoided crossing, the CCSD similarity-transformed
Hamiltonian is defective, meaning that it has complex eigenvalues and/or
negative oscillator strengths. When an external field is applied to drive
transitions to states exhibiting these traits, unphysical dynamics are
observed. For example, the stationary states that make up the time-dependent
state acquire populations that can be negative, exceed one, or even be
complex-valued