Electronic Absorption and Resonance Raman Spectroscopy from Ab Initio Quantum Molecular Dynamics

196

245

An atomic orbital-based formulation of analytical gradients and nonadiabatic coupling vector elements for the state-averaged complete active space self-consistent field method on graphical processing units We present a new algorithm for ab initio quantum nonadiabatic molecular dynamics that combines the best features of ab initio Multiple Spawning (AIMS) and Multiconfigurational Ehrenfest (MCE) methods. In this new method, ab initio multiple cloning (AIMC), the individual trajectory basis functions (TBFs) follow Ehrenfest equations of motion (as in MCE). However, the basis set is expanded (as in AIMS) when these TBFs become sufficiently mixed, preventing prolonged evolution on an averaged potential energy surface. We refer to the expansion of the basis set as "cloning," in analogy to the "spawning" procedure in AIMS. This synthesis of AIMS and MCE allows us to leverage the benefits of mean-field evolution during periods of strong nonadiabatic coupling while simultaneously avoiding mean-field artifacts in Ehrenfest dynamics. We explore the use of time-displaced basis sets, "trains," as a means of expanding the basis set for little cost. We also introduce a new bra-ket averaged Taylor expansion (BAT) to approximate the necessary potential energy and nonadiabatic coupling matrix elements. The BAT approximation avoids the necessity of computing electronic structure information at intermediate points between TBFs, as is usually done in saddle-point approximations used in AIMS. The efficiency of AIMC is demonstrated on the nonradiative decay of the first excited state of ethylene. The AIMC method has been implemented within the AIMS-MOLPRO package, which was extended to include Ehrenfest basis functions.

Section: Discussionmentioning

confidence: 99%

Ab initio multiple cloning algorithm for quantum nonadiabatic molecular dynamics

Makhov

Glover

Martı́nez

et al. 2014

196

245

“…AIMS in combination with ab initio complete active space self-consistent field theory has been used to calculate the vibronic structure of the S 0 → S 1 absorption spectrum of ethylene. 12 However, these approaches are also approximate since they are not converged with respect to the size of the basis set and matrix elements of the Hamiltonian are evaluated only approximately.…”

mentioning

confidence: 99%

Semiclassical on-the-fly computation of the S→S1 absorption spectrum of formaldehyde

Tatchen

Pollak

2009

The anharmonic S0→S1 vibronic absorption spectrum of the formaldehyde molecule is computed on the fly using semiclassical dynamics. This first example of an on-the-fly semiclassical computation of a vibronic spectrum was achieved using a unit prefactor modified frozen Gaussian semiclassical propagator for the excited state. A sample of 6000 trajectories sufficed for obtaining a converged spectrum, which is in reasonable agreement with experiment. Similar agreement is not obtained when using a harmonic approximation for the spectrum, demonstrating the need for a full anharmonic computation. This first example provides a resolution of ∼100 cm−1. Potential ways of improving the methodology and obtaining higher resolution and accuracy are discussed.

“…44 The former study based on the wavefunction theories also predicted the two lowest bright states at approximately 6 eV and 7.8 eV, and provided the assignment as π → 3s and π → π * excitations, respectively. 45,46 While the wavefunction theory provides the delocalized Rydberg state properly, the current study underestimates the delocalization of electron density even though the excitation energy is correct. This localized nature shown in the calculated density difference map results from the well-known TDDFT issue that it cannot suitably describe Rydberg excitations.…”

Section: B Linear Absorption Spectrummentioning

confidence: 60%

“…The constrained-nuclear-vibration coupled electronic absorption spectra with CH stretching (black), CC stretching (red), CH wagging (blue), and CH twisting (green) excitations, respectively. multiple spawning methods which can treat nuclei as quantum particles, 31,45,46 the study based on the classical dynamics also shows a good agreement with experimental results since the absorption spectrum of an ethylene molecule is determined in the potential energy surfaces which is distant from conical intersections. In the case that the non-adiabaticity is important, the classical treatment of nuclear motions would be questionable.…”

Section: The Effect Of Nucleus Motion On Absorption Spectrummentioning

confidence: 69%

Efficient electron dynamics with the planewave-based real-time time-dependent density functional theory: Absorption spectra, vibronic electronic spectra, and coupled electron-nucleus dynamics

Min¹,

Cho²,

Kim³

2011

Articles you may be interested inA time-dependent density-functional theory and complete active space self-consistent field method study of vibronic absorption and emission spectra of coumarin J. Chem. Phys. 141, 014306 (2014) The electron dynamics with complex third-order Suzuki-Trotter propagator (ST 3 ) has been implemented into a planewave (PW) based density functional theory program, and several applications including linear absorption spectra and coupled electron-nucleus dynamics have been calculated. Since the ST 3 reduces the number of Fourier transforms to less than half compared to the fourthorder Suzuki-Trotter propagator (ST 4 ), more than twice faster calculations are possible by exploiting the ST 3 . We analyzed numerical errors of both the ST 3 and the ST 4 in the presence/absence of an external field for several molecules such as Al 2 , N 2 , and C 2 H 4 . We obtained that the ST 3 gives the same order of numerical errors (10 −5 Ry after 100 fs) as the ST 4 . Also, the time evolution of dipole moments, hence the absorption spectrum, is equivalent for both ST 3 and ST 4 . As applications, the linear absorption spectrum for an ethylene molecule was studied. From the density difference analysis, we showed that the absorption peaks at 6.10 eV and 7.65 eV correspond to the π → 4a g and π → π * excitation bands, respectively. We also investigated the molecular vibrational effect to the absorption spectra of an ethylene molecule and the dynamics of a hydrogen molecule after the σ → σ * transition by formulating coupled electron-nucleus dynamics within the Ehrenfest regime. The trajectory of nuclei follows the excited state potential energy curve exactly.