6 research outputs found
Reaction Energetics for the Abstraction Process C<sub>2</sub>H<sub>3</sub> + H<sub>2</sub> → C<sub>2</sub>H<sub>4</sub> + H
The fundamentally important combustion reaction of vinyl radical with hydrogen has been studied in the laboratory by at least five experimental groups. Herein, the reaction C<sub>2</sub>H<sub>3</sub> + H<sub>2</sub> → C<sub>2</sub>H<sub>4</sub> + H has been examined using focal-point analysis. Molecular energies were determined from extrapolations to the complete basis-set limit using correlation-consistent basis sets (cc-pVTZ, cc-pVQZ, and cc-pV5Z) and coupled-cluster theory with single and double excitations (CCSD), perturbative triples [CCSD(T)], full triples [CCSDT], and perturbative quadruples [CCSDT(Q)]. Reference geometries were optimized at the all-electron CCSD(T)/cc-pCVQZ level. Computed energies were also corrected for relativistic effects and the Born–Oppenheimer approximation. The activation energy for hydrogen abstraction is predicted to be 9.65 kcal mol<sup>–1</sup>, and the overall reaction is predicted to be exothermic by 5.65 kcal mol<sup>–1</sup>. Natural resonance theory (NRT) analysis was performed to verify the reaction pathway and describe bond-breaking and bond-forming events along the reaction coordinate
Structural Distortions Accompanying Noncovalent Interactions: Methane–Water, the Simplest C–H Hydrogen Bond
Neglect
of fragment structural distortions resulting from noncovalent
interactions is a common practice when examining a potential energy
surface (PES). Herein, we make quantitative predictions concerning
the magnitude of such distortions in the methane–water system.
Coupled cluster methods up to perturbative quadruples [CCSDTÂ(Q)] were
used in the structural optimizations to the complete basis set limit
(using up to cc-pV6Z basis sets). Our results show that the interaction
energy differences between the fully optimized and nonoptimized structures
are on the order of 0.02 kcal mol<sup>–1</sup>. These findings
imply that scanning the PES of a very weakly bound noncovalent system,
while neglecting intramolecular distortions, is a reasonable approximation
for points other than the minima
Analytic Energy Gradients for Variational Two-Electron Reduced-Density-Matrix-Driven Complete Active Space Self-Consistent Field Theory
Analytic energy gradients are presented
for a variational two-electron
reduced-density-matrix (2-RDM)-driven complete active space self-consistent
field (CASSCF) method. The active-space 2-RDM is determined using
a semidefinite programing (SDP) algorithm built upon an augmented
Lagrangian formalism. Expressions for analytic gradients are simplified
by the fact that the Lagrangian is stationary with respect to variations
in both the primal and the dual solutions to the SDP problem. Orbital
response contributions to the gradient are identical to those that
arise in conventional CASSCF methods in which the electronic structure
of the active space is described by a full configuration interaction
(CI) wave function. We explore the relative performance of variational
2-RDM (v2RDM)- and CI-driven CASSCF for the equilibrium geometries
of 20 small molecules. When enforcing two-particle <i>N</i>-representability conditions, full-valence v2RDM-CASSCF-optimized
bond lengths display a mean unsigned error of 0.0060 Ă… and a
maximum unsigned error of 0.0265 Ă…, relative to those obtained
from full-valence CI-CASSCF. When enforcing partial three-particle <i>N</i>-representability conditions, the mean and maximum unsigned
errors are reduced to only 0.0006 and 0.0054 Ă…, respectively.
For these same molecules, full-valence v2RDM-CASSCF bond lengths computed
in the cc-pVQZ basis set deviate from experimentally determined ones
on average by 0.017 and 0.011 Ă… when enforcing two- and three-particle
conditions, respectively, whereas CI-CASSCF displays an average deviation
of 0.010 Ă…. The v2RDM-CASSCF approach with two-particle conditions
is also applied to the equilibrium geometry of pentacene; optimized
bond lengths deviate from those derived from experiment, on average,
by 0.015 Ă… when using a cc-pVDZ basis set and a (22e,22o) active
space
Simulation of the VUV Absorption Spectra of Oxygenates and Hydrocarbons: A Joint Theoretical–Experimental Study
Vacuum UV absorption spectroscopy is regularly used to
provide
unambiguous identification of a target species, insight into the electronic
structure of molecules, and quantitative species concentrations. As
molecules of interest have become more complex, theoretical spectra
have been used in tandem with laboratory spectroscopic analysis or
as a replacement when experimental data is unavailable. However, it
is difficult to determine which theoretical methodologies can best
simulate experiment. This study examined the performance of EOM-CCSD
and 10 TD-DFT functionals (B3LYP, BH&HLYP, BMK, CAM-B3LYP, HSE,
M06-2X, M11, PBE0, ωB97X-D, and X3LYP) to produce reliable vacuum
UV absorption spectra for 19 small oxygenates and hydrocarbons using
vertical excitation energies. The simulated spectra were analyzed
against experiment using both a qualitative analysis and quantitative
metrics, including cosine similarity, relative integral change, mean
signed error, and mean absolute error. Based on our ranking system,
it was determined that M06-2X was consistently the top performing
TD-DFT method with BMK, CAM-B3LYP, and ωB97X-D also producing
reliable spectra for these small combustion species
Simulation of the VUV Absorption Spectra of Oxygenates and Hydrocarbons: A Joint Theoretical–Experimental Study
Vacuum UV absorption spectroscopy is regularly used to
provide
unambiguous identification of a target species, insight into the electronic
structure of molecules, and quantitative species concentrations. As
molecules of interest have become more complex, theoretical spectra
have been used in tandem with laboratory spectroscopic analysis or
as a replacement when experimental data is unavailable. However, it
is difficult to determine which theoretical methodologies can best
simulate experiment. This study examined the performance of EOM-CCSD
and 10 TD-DFT functionals (B3LYP, BH&HLYP, BMK, CAM-B3LYP, HSE,
M06-2X, M11, PBE0, ωB97X-D, and X3LYP) to produce reliable vacuum
UV absorption spectra for 19 small oxygenates and hydrocarbons using
vertical excitation energies. The simulated spectra were analyzed
against experiment using both a qualitative analysis and quantitative
metrics, including cosine similarity, relative integral change, mean
signed error, and mean absolute error. Based on our ranking system,
it was determined that M06-2X was consistently the top performing
TD-DFT method with BMK, CAM-B3LYP, and ωB97X-D also producing
reliable spectra for these small combustion species
Psi4NumPy: An Interactive Quantum Chemistry Programming Environment for Reference Implementations and Rapid Development
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<p><i>Psi4NumPy</i> demonstrates the use of efficient computational kernels from the open-
source <i>Psi4</i> program through the popular <i>NumPy</i> library for linear algebra in Python
to facilitate the rapid development of clear, understandable Python computer code for
new quantum chemical methods, while maintaining a relatively low execution time. Using these tools, reference implementations have been created for a number of methods,
including self-consistent field (SCF), SCF response, many-body perturbation theory,
coupled-cluster theory, configuration interaction, and symmetry-adapted perturbation
theory. Further, several reference codes have been integrated into Jupyter notebooks,
allowing background and explanatory information to be associated with the imple-
mentation. <i>Psi4NumPy</i> tools and associated reference implementations can lower the
barrier for future development of quantum chemistry methods. These implementa-
tions also demonstrate the power of the hybrid C++/Python programming approach
employed by the <i>Psi4</i> program. </p>
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