Diabatic States of Molecules

Abstract

Quantitative simulations of electronically nonadiabatic molecular processes require both accurate dynamics algorithms and accurate electronic structure information. Direct semiclassical nonadiabatic dynamics is expensive due to the high cost of electronic structure calculations, and hence it is limited to small systems, limited ensemble averaging, ultrafast processes, and/or electronic structure methods that are only semiquantitatively accurate. The cost of dynamics calculations can be made manageable if analytic fits are made to the electronic structure data, and such fits are most conveniently carried out in a diabatic representation because the surfaces are smooth and the couplings between states are smooth scalar functions. Diabatic representations, unlike the adiabatic ones produced by most electronic structure methods, are not unique, and finding suitable diabatic representations often involves time-consuming nonsystematic diabatization steps. The biggest drawback of using diabatic bases is that it can require large amounts of effort to perform a globally consistent diabatization, and one of our goals has been to develop methods to do this efficiently and automatically. In this Feature Article, we introduce the mathematical framework of diabatic representations, and we discuss diabatization methods, including adiabatic-to-diabatic transformations and recent progress toward the goal of automatization.

Internal conversion and intersystem crossing dynamics based on coupled potential energy surfaces with full geometry-dependent spin–orbit and derivative couplings. Nonadiabatic photodissociation dynamics of NH3(A) leading to the NH(X3Σ−, a1Δ) + H2 channel

“…[1][2][3][4] Such processes are ubiquitous in nature and understanding them requires quality simulation methods. [5][6][7][8][9][10][11][12][13][14] In this work, we focus on the first principle determination of the molecular product channel of the nonadiabatic photodissociation…”

“…1–4 Such processes are ubiquitous in nature and understanding them requires quality simulation methods. 5–14 In this work, we focus on the first principle determination of the molecular product channel of the nonadiabatic photodissociationInternal conversion (1a) and intersystem crossing (1b) are the two major mechanisms of nonradiative decay, with the former involving states with same spin multiplicity and induced by conical intersections and the latter involving states with different spin multiplicities and induced by spin-orbit coupling (SOC). 15–17 Recently, there has been significant interest in competition between these two mechanisms.…”

Representation of Diabatic Potential Energy Matrices for Multiconfiguration Time-Dependent Hartree Treatments of High-Dimensional Nonadiabatic Photodissociation Dynamics

“…Theoretically, photodissociation dynamics can be characterized by solving the nuclear Schrödinger equation on a single or multiple coupled potential energy surfaces (PESs). , In recent years, significant advances have been made in constructing PESs and multi-dimensional diabatic potential energy matrices (DPEMs) from high-level ab initio calculations, − which have enabled quantum mechanical characterization of nonadiabatic photodissociation dynamics of important prototypes. , However, such first-principles description of photodissociation dynamics, particularly that based on an expansion on a grid or a basis, remains challenging for high-dimensional systems because the computational costs (both memory and CPU time) increase exponentially with the size of the problem. Because of this so-called curse of dimensionality, most such quantum mechanical investigations of photodissociation dynamics have been restricted to low (<9) dimensions, which may not be sufficient to describe dynamics quantitatively in molecules with more than four atoms.…”

“…It should be pointed out that the MCCPD method relies on an existing global PES or DPEM, which is a major difference compared to the machine learning method by Manzhos and Carrington, which directly turns the ab initio points into a PES with the SOP form. However, we note that the fitting of a global PES or DPEM is more a less a solved problem, − where the computational costs is dominated by the expensive ab initio calculations. The gain of this extra step of fitting the global PES/DPEM first is obvious that one can have an overall idea about the quality of different regions described by the PES/DPEM.…”

Multi-state and Multi-mode Vibronic Coupling Effects in the Photoionization Spectroscopy of Acetaldehyde

“…Therefore, although not strictly governed by the selection rule, coupling among them is expected to have impact on the dynamics. Such couplings are rare and treated in different ways by various practitioners in the literature. − Generally, this type of vibronic coupling often occurs in molecules belonging to the equilibrium point group of low symmetry. ,, The vibronic coupling between the electronic states of the same spatial symmetry in the present case is calculated by applying the fourfold way diabatization scheme proposed by Truhlar et al − at the level of the multi-configuration quasi-degenerate perturbation theory (MCQDPT). − …”