Fast Screening of Minimum Energy Crossing Points with Semiempirical Tight-Binding Methods

Abstract

The investigation of photochemical processes is a highly active field in computational chemistry. One research direction is the automated exploration and identification of minimum energy conical intersection (MECI) geometries. However, due to the immense technical effort required to calculate nonadiabatic potential energy landscapes, the routine application of such computational protocols is severely limited. In this study, we will discuss the prospect of combining adiabatic potential energy surfaces from semiempirical quantum mechanical calculations with specialized confinement potential and metadynamics simulations to identify S0/T1 minimum energy crossing point (MECP) geometries. It is shown that MECPs calculated at the GFN2-xTB level can provide suitable approximations to high-level S0/S1 ab initio conical intersection geometries at a fraction of the computational cost. Reference MECIs of benzene are studied to illustrate the basic concept. An example application of the presented protocol is demonstrated for a set of photoswitch molecules.

Finding Excited-State Minimum Energy Crossing Points on a Budget: Non-Self-Consistent Tight-Binding Methods

“…An in-depth discussion for these systems can be found in previous work and the citing literature. ,, To summarize the current findings, GFN0-xTB shows virtually identical characteristics as the related self-consistent GFN2-xTB method in the description of GS/OS1 MECPs, which clearly resemble the S 0 / S 1 MECI between adiabatic states. RMSDs at the GFN0-xTB level compared to the reference structures are marginally worse than those with GFN2-xTB, as would be expected for this more approximate non-self-consistent method.…”

“…As in previous work, we are pursuing a “derivative coupling vector-free” treatment ,− that provides MECPs of two (or more) diabatic multidimensional PESs. These points can then serve as approximations to MECIs between adiabatic states that are obtained from multideterminant wave functions .…”

“…A key feature of this procedure, and the penalty functions in particular, is their ability to locate transition state analogues, i.e., the MECP, involving discontinuous first and second derivatives in the adiabatic framework . In eq , a total of three empirical parameters, σ, k , and α, are chosen to converge the energy gap to zero near the seam region while providing a linearly growing energy bias far away from it. , Detailed adjustment of the empirical parameters in the penalty potential may be required for some systems, but manually selected values of σ = 10.0, α = 0.005, and k = 0.25 often work well and were used throughout this study. While the σ and α parameters were chosen in accordance with earlier studies, , k was selected to provide the linearly growing bias far away from the potential minimum.…”

“…The electronic temperature T el serves as a simple adjustment parameter to steer the amount of smeared occupations and can often be set to a rather high value. Note that no electronic entropy term − T el S el is calculated from the FON for this specialized application of GFN0-xTB, which in contrast to regular xTB schemes ensures that eq approaches zero close to the MECP. , In summary, the procedure employs GFN0-xTB (eq ) to calculate energies and gradients for multiple diabatic states. The latter are obtained from a single diagonalization of the Hamiltonian matrix using different orbital occupations.…”

Multifaceted View on the Mechanism of a Photochemical Deracemization Reaction

“…The preceding ISC from T 1 to the open shell singlet is readily feasible, since for (−)- 3 · 6b , the two states are found to be isoenergetic [−106.9 kJ mol –1 relative to (−)- 2 · ( R )- 1b ] and structurally indistinguishable. The energetically lowest S 0 / S 1 minimum energy conical intersection (MECI) could be identified by a recently developed sampling algorithm (see the Supporting Information for details). Remarkably, among all sampled structures, no precursor geometry could be identified that would correspond to a direct HAT to the prostereogenic carbon atom of radical intermediate 6b .…”