5 research outputs found
α‑CASSCF: An Efficient, Empirical Correction for SA-CASSCF To Closely Approximate MS-CASPT2 Potential Energy Surfaces
Because of its computational
efficiency, the state-averaged complete
active-space self-consistent field (SA-CASSCF) method is commonly
employed in nonadiabatic ab initio molecular dynamics. However, SA-CASSCF
does not effectively recover dynamical correlation. As a result, there
can be qualitative differences between SA-CASSCF potential energy
surfaces (PESs) and more accurate reference surfaces computed using
multistate complete active space second-order perturbation theory
(MS-CASPT2). Here we introduce an empirical correction to SA-CASSCF
that scales the splitting between individual states and the state-averaged
energy. We call this the α-CASSCF method, and we show here that
it significantly improves the accuracy of relative energies and PESs
compared with MS-CASPT2 for the chromophores of green fluorescent
and photoactive yellow proteins. As such, this method may prove to
be quite valuable for nonadiabatic dynamics
Quantum-Mechanical Analysis of the Energetic Contributions to π Stacking in Nucleic Acids versus Rise, Twist, and Slide
Symmetry-adapted perturbation theory (SAPT) is applied
to pairs
of hydrogen-bonded nucleobases to obtain the energetic components
of base stacking (electrostatic, exchange-repulsion, induction/polarization,
and London dispersion interactions) and how they vary as a function
of the helical parameters Rise, Twist, and Slide. Computed average
values of Rise and Twist agree well with experimental data for B-form
DNA from the Nucleic Acids Database, even though the model computations
omitted the backbone atoms (suggesting that the backbone in B-form
DNA is compatible with having the bases adopt their ideal stacking
geometries). London dispersion forces are the most important attractive
component in base stacking, followed by electrostatic interactions.
At values of Rise typical of those in DNA (3.36 Å), the electrostatic
contribution is nearly always attractive, providing further evidence
for the importance of charge-penetration effects in π–π
interactions (a term neglected in classical force fields). Comparison
of the computed stacking energies with those from model complexes
made of the “parent” nucleobases purine and 2-pyrimidone
indicates that chemical substituents in DNA and RNA account for 20–40%
of the base-stacking energy. A lack of correspondence between the
SAPT results and experiment for Slide in RNA base-pair steps suggests
that the backbone plays a larger role in determining stacking geometries
in RNA than in B-form DNA. In comparisons of base-pair steps with
thymine versus uracil, the thymine methyl group tends to enhance the
strength of the stacking interaction through a combination of dispersion
and electrosatic interactions
An Ab Initio Exciton Model Including Charge-Transfer Excited States
The Frenkel exciton model is a useful
tool for theoretical studies
of multichromophore systems. We recently showed that the exciton model
could be used to coarse-grain electronic structure in multichromophoric
systems, focusing on singly excited exciton states [Acc. Chem. Res. 2014, 47, 2857−2866]. However, our previous implementation excluded charge-transfer
excited states, which can play an important role in light-harvesting
systems and near-infrared optoelectronic materials. Recent studies
have also emphasized the significance of charge-transfer in singlet
fission, which mediates the coupling between the locally excited states
and the multiexcitonic states. In this work, we report on an ab initio
exciton model that incorporates charge-transfer excited states and
demonstrate that the model provides correct charge-transfer excitation
energies and asymptotic behavior. Comparison with TDDFT and EOM-CC2
calculations shows that our exciton model is robust with respect to
system size, screening parameter, and different density functionals.
Inclusion of charge-transfer excited states makes the exciton model
more useful for studies of singly excited states and provides a starting
point for future construction of a model that also includes double-exciton
states
Tensor Hypercontraction Second-Order Møller–Plesset Perturbation Theory: Grid Optimization and Reaction Energies
We
have recently introduced the tensor hypercontraction (THC) method
for electronic structure, including MP2. Here, we present an algorithm
for THC-MP2 that lowers the memory requirements as well as the prefactor
while maintaining the formal quartic scaling that we demonstrated
previously. We also describe a procedure to optimize quadrature grids
used in grid-based least-squares (LS) THC-MP2. We apply this algorithm
to generate grids for first-row atoms with less than 100 points/atom
while incurring negligible errors in the computed energies. We benchmark
the LS-THC-MP2 method using optimized grids for a wide variety of
tests sets including conformational energies and reaction barriers
in both the cc-pVDZ and cc-pVTZ basis sets. These tests demonstrate
that the THC methodology is not limited to small basis sets and that
it incurs negligible errors in both absolute and relative energies
Estimation of Electrostatic Interaction Energies on a Trapped-Ion Quantum Computer
We present the first
hardware implementation of electrostatic interaction
energies by using a trapped-ion quantum computer. As test system for
our computation, we focus on the reduction of NO to N2O
catalyzed by a nitric oxide reductase (NOR). The quantum computer
is used to generate an approximate ground state within the NOR active
space. To efficiently measure the necessary one-particle density matrices,
we incorporate fermionic basis rotations into the quantum circuit
without extending the circuit length, laying the groundwork for further
efficient measurement routines using factorizations. Measurements
in the computational basis are then used as inputs for computing the
electrostatic interaction energies on a classical computer. Our experimental
results strongly agree with classical noise-less simulations of the
same circuits, finding electrostatic interaction energies within chemical
accuracy despite hardware noise. This work shows that algorithms tailored
to specific observables of interest, such as interaction energies,
may require significantly fewer quantum resources than individual
ground state energies would require in the straightforward supermolecular
approach