54 research outputs found
How Do Methyl Groups Enhance the Triplet Chemiexcitation Yield of Dioxetane?
Chemiluminescence is the emission of light as a result of a nonadiabatic
chemical reaction. The present work is concerned with understanding the yield
of chemiluminescence, in particular how it dramatically increases upon
methylation of 1,2-dioxetane. Both ground-state and nonadiabatic dynamics
(including singlet excited states) of the decomposition reaction of various
methyl-substituted dioxetanes have been simulated. Methyl-substitution leads to
a significant increase in the dissociation time scale. The rotation around the
O-C-C-O dihedral angle is slowed; thus, the molecular system stays longer in
the "entropic trap" region. A simple kinetic model is proposed to explain how
this leads to a higher chemiluminescence yield. These results have important
implications for the design of efficient chemiluminescent systems in medical,
environmental, and industrial applications
Anomalous Magnetism of Uranium(IV)-Oxo and -Imido Complexes Reveals Unusual Doubly- Degenerate Electronic Ground States
The OpenMolcas Web: A Community-Driven Approach to Advancing Computational Chemistry
The developments of the open-source OpenMolcas chemistry software environment since spring 2020 are described, with a focus on novel functionalities accessible in the stable branch of the package or via interfaces with other packages. These developments span a wide range of topics in computational chemistry and are presented in thematic sections: electronic structure theory, electronic spectroscopy simulations, analytic gradients and molecular structure optimizations, ab initio molecular dynamics, and other new features. This report offers an overview of the chemical phenomena and processes OpenMolcas can address, while showing that OpenMolcas is an attractive platform for state-of-the-art atomistic computer simulations
Smooth Things Come in Threes: A Diabatic Surrogate Model for Conical Intersection Optimization
The optimization of conical intersection structures is complicated by the non-differentiability of the adiabatic potential energy surfaces. In this work, we build a pseudo-diabatic surrogate model, based on Gaussian process regression, formed by three smooth and differentiable surfaces that can adequately reproduce the adiabatic surfaces. Using this model with the restricted variance optimization method results in a notable decrease of the overall computational effort required to obtain minimum energy crossing points
Multiconfigurational quantum chemistry: The CASPT2 method
This chapter presents the theory behind the CASPT2 method and its adaptation
to a multi-state formalism. The chapter starts with an introduction of the
theory of the CASPT2 method - an application of Rayleigh-Schr\"odinger
perturbation theory applied to multiconfigurational reference function - as it
was originally presented. In particular, we discuss the nature of the reference
Hamiltonian and the first-order interacting space. This is followed by some
detailed discussion with respect to the intruder state problem and various
shift techniques to address this problem. Afterwards a longer review on
alternative reference Hamiltonians, which to some degree or completely remove
the intruder state problem, is put forward. Subsequently the presently proposed
multi-state versions of the CASPT2 method are discussed in some detail. The
chapter is concluded with a review of different benchmark assessments of the
accuracy of the method and a qualified suggestion on the future development
potentials of the approach.Comment: This is a post-peer-review, pre-copyedit version of a chapter
accepted and published in the book "Theoretical and Computational
Photochemistry
Regularized CASPT2: an intruder-state-free approach
In this work we present a new approach to fix the intruder state problem (ISP) in CASPT2 based on Ïp regularization. The resulting -CASPT2 method is compared to previous techniques, namely the real and imaginary level shifts, on a theoretical basis and by performing a series of systematic calculations. The analysis is focused on two aspects, the effectiveness of -CASPT2 in removing the ISP and the sensitivity of the approach with respect to the input parameter. We found that Ïp-CASPT2 compares favorably with respect to previous approaches, and that different versions, -CASPT2 and -CASPT2, have different potential application domains. This analysis also reveals the unsuitability of the real level shift technique as a general way to avoid the intruder state problem
Uncontracted basis sets for ab initio calculations of muonic atoms and molecules
In this work, we investigated muonic atoms and molecules from a quantum chemist's viewpoint by incorporating muons in the CASSCF model. With the aim of predicting muonic X-ray energies, primitive muonic basis sets were developed for a selection of elements. The basis sets were then used in CASSCF calculations of various atoms and molecules to calculate muonic excited states. We described the influence of nuclear charge distribution in predicting muonic X-ray energies. Effects of the electronic wave function on the muonic X-ray energies were also examined. We have computationally demonstrated how the muon can act as a probe for the nuclear charge distribution or electronic wave function by considering lower or higher muonic excited states, respectively
Quantum mechanical methods applied to excitation energy transfer: A comparative analysis on excitation energies and electronic couplings
We present a comparative study on the influence of the quantum mechanical QM method including basis set on the evaluation of transition energies, transition densities and dipoles, and
excitation energy transfer EET electronic couplings for a series of chromophores and the corresponding pairs typically found in organic electro-optical devices and photosynthetic systems. On these systems we have applied five different QM levels of description of increasing accuracy ZINDO, CIS, TD-DFT, CASSCF, and SAC-CI. In addition, we have tested the effects of a surrounding environment either mimicking a solvent or a protein matrix on excitation energies, transition dipoles, and electronic couplings through the polarizable continuum model PCM
description. Overall, the results obtained suggest that the choice of the QM level of theory affects the electronic couplings much less than it affects excitation energies. We conclude that reasonable estimates can be obtained using moderate basis sets and inexpensive methods such as configuration interaction of single excitations or time-dependent density functional theory when appropriately
coupled to realistic solvation models such as PCM
The versatility of Cholesky decomposition in electronic structure theory
The resolution-of-the-identity (RI) or density fitting (DF) approximation for the electron repulsion integrals (ERIs) has become a standard component of accelerated and reduced-scaling implementations of first-principles Gaussian- type orbital electronic-structure methods. The Cholesky decomposition (CD) of the ERIs has also become increasingly deployed across quantum chemistry packages in the last decade, even though its early applications were mostly limited to high-accuracy methods such as coupled-cluster theory and multi-configurational approaches. Starting with a summary of the basic theory underpinning both the CD and RI/DF approximations, thus underlining the extremely close relation of the CD and RI/DF techniques, we provide a brief and largely chronological review of the evolution of the CD approach from its birth in 1977 to its current state. In addition to being a purely numerical procedure for handling ERIs, thus providing robust and computationally efficient approximations to the exact ERIs that have been found increasingly useful on modern computer platforms, CD also offers highly accurate approaches for generating auxiliary basis sets for the RI/DF approximation on the fly, as the two approaches have a deep mathematical connection. In this review, we aim to provide a concise reference of the main techniques employed in various CD approaches in electronic structure theory, to exemplify the connection between the CD and RI/DF approaches, and to clarify the state of the art to guide new implementations of CD approaches across electronic structure programs
- âŠ