scholarly journals Communication: A mean field platform for excited state quantum chemistry

2018 ◽  
Vol 149 (8) ◽  
pp. 081101 ◽  
Author(s):  
Jacqueline A. R. Shea ◽  
Eric Neuscamman
Keyword(s):  
Excited State ◽  
Quantum Chemistry ◽  
Mean Field

Excited-State Vibronic Dynamics of Bacteriorhodopsin from Two-Dimensional Electronic Photon Echo Spectroscopy and Multiconfigurational Quantum Chemistry

2020 ◽  
Vol 11 (10) ◽  
pp. 3889-3896
Author(s):  
Samer Gozem ◽  
Philip J. M. Johnson ◽  
Alexei Halpin ◽  
Hoi Ling Luk ◽  
Takefumi Morizumi ◽  
...  
Keyword(s):  
Excited State ◽  
Quantum Chemistry ◽  
Photon Echo ◽  
Two Dimensional

scholarly journals Richardson–Gaudin mean-field for strong correlation in quantum chemistry

2020 ◽  
Vol 153 (10) ◽  
pp. 104110 ◽  
Author(s):  
Paul A. Johnson ◽  
Charles-Émile Fecteau ◽  
Frédéric Berthiaume ◽  
Samuel Cloutier ◽  
Laurie Carrier ◽  
...  
Keyword(s):  
Quantum Chemistry ◽  
Strong Correlation ◽  
Mean Field

scholarly journals Tracking the excited-state time evolution of the visual pigment with multiconfigurational quantum chemistry

2007 ◽  
Vol 104 (19) ◽  
pp. 7764-7769 ◽  
Author(s):  
L. M. Frutos ◽  
T. Andruniow ◽  
F. Santoro ◽  
N. Ferre ◽  
M. Olivucci
Keyword(s):  
Excited State ◽  
Quantum Chemistry ◽  
Time Evolution ◽  
Visual Pigment ◽  
State Time

Comparison of the mean field and Bohmian semi-classical approximations to the Rabi model

2021 ◽  
pp. 2150270
Author(s):  
Dirk-André Deckert ◽  
Leopold Kellers ◽  
Travis Norsen ◽  
Ward struyve
Keyword(s):  
Quantum Mechanics ◽  
Excited State ◽  
Measurement Problem ◽  
Mean Field ◽  
Bohmian Mechanics ◽  
Back Reaction ◽  
Standard Quantum ◽  
Level Atom ◽  
Classical Approximation ◽  
Rabi Model

Bohmian mechanics is an alternative to standard quantum mechanics that does not suffer from the measurement problem. While it agrees with standard quantum mechanics concerning its experimental predictions, it offers novel types of approximations not suggested by the latter. Of particular interest are semi-classical approximations, where part of the system is treated classically. Bohmian semi-classical approximations have been explored before for systems without electromagnetic interactions. Here, the Rabi model is considered as a simple model involving light-matter interaction. This model describes a single mode electromagnetic field interacting with a two-level atom. As is well-known, the quantum treatment and the semi-classical treatment (where the field is treated classically rather than quantum mechanically) give qualitatively different results. We analyze the Rabi model using a different semi-classical approximation based on Bohmian mechanics. In this approximation, the back-reaction from the two-level atom onto the classical field is mediated by the Bohmian configuration of the two-level atom. We find that the Bohmian semi-classical approximation gives results comparable to the usual mean field one for the transition between ground and first excited state. Both semi-classical approximations tend to reproduce the collapse of the population inversion, but fail to reproduce the revival, which is characteristic of the full quantum description. Also an example of a higher excited state is presented where the Bohmian approximation does not perform so well.

Correlation Methods

2007 ◽  
Author(s):  
Kenneth G. Dyall ◽  
Knut Faegri
Keyword(s):  
Quantum Chemistry ◽  
Time Reversal ◽  
Relativistic Quantum ◽  
Mean Field ◽  
Spin Symmetry ◽  
Nonrelativistic Quantum ◽  
Field Methods ◽  
Hartree Fock ◽  
The Mean ◽  
The Difference

It is well known from nonrelativistic quantum chemistry that mean-field methods, such as the Hartree–Fock (HF) model, provide mainly qualitative insights into the electronic structure and bonding of molecules. To obtain reliable results of “chemical accuracy” usually requires models that go beyond the mean field and account for electron correlation. There is no reason to expect that the mean-field approach should perform significantly better in this respect for the relativistic case, and so we are led to develop schemes for introducing correlation into our models for relativistic quantum chemistry. There is no fundamental change in the concept of correlation between relativistic and nonrelativistic quantum chemistry: in both cases, correlation describes the difference between a mean-field description, which forms the reference state for the correlation method, and the exact description. We can also define dynamical and nondynamical correlation in both cases. There is in fact no formal difference between a nonrelativistic spin–orbital-based formalism and a relativistic spinor-based formalism. Thus we should be able to transfer most of the schemes for post-Hartree–Fock calculations to a relativistic post-Dirac–Hartree–Fock model. Several such schemes have been implemented and applied in a range of calculations. The main technical differences to consider are those arising from having to deal with integrals that are complex, and the need to replace algorithms that exploit the nonrelativistic spin symmetry by schemes that use time-reversal and double-group symmetry. In addition to these technical differences, however, there are differences of content between relativistic and nonrelativistic methods. The division between dynamical and nondynamical correlation is complicated by the presence of the spin–orbit interaction, which creates near-degeneracies that are not present in the nonrelativistic theory. The existence of the negative-energy states of relativistic theory raise the question of whether they should be included in the correlation treatment. The first two sections of this chapter are devoted to a discussion of these issues. The main challenges in the rest of this chapter are to handle the presence of complex integrals and to exploit time-reversal symmetry.

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