Related Experiment Video
Updated: Aug 5, 2026
09:00
Experimental Methods for Spin- and Angle-Resolved Photoemission Spectroscopy Combined with Polarization-Variable Laser
Published on: June 28, 2018
SURFXsim: Free-Energy Simulations of Spin-Forbidden Events with a Spin-Gap Collective Variable
Ajmal Rahman Mullukkandy1,2, Bernd Ensing3, Carles Bo1,2
1Department of Physical and Inorganic Chemistry, Universitat Rovira i Virgili, Marcel lí Domingo, 1, 43007Tarragona, Spain.
Journal of Chemical Theory and Computation
|July 27, 2026
Summary
Simulating spin-forbidden chemical reactions is now more accessible. The new SURFXsim framework combines molecular dynamics with enhanced sampling for studying these challenging processes in larger systems.
Area of Science:
- Computational chemistry
- Chemical dynamics
- Quantum mechanics
Background:
- Spin-forbidden processes are crucial in catalysis and photochemistry.
- Simulating these events is computationally challenging due to transitions between different spin states.
- Current methods offer static views or are too demanding for large systems.
Purpose of the Study:
- To develop a practical computational framework for simulating spin-forbidden events.
- To enable the study of these processes in medium- and large-sized chemical systems.
- To provide a flexible tool for exploring spin-crossing phenomena.
Main Methods:
- Atomistic molecular dynamics combined with a configuration-dependent mixed Hamiltonian.
- Enhanced sampling using metadynamics along a spin-gap collective variable.
- Integration with machine-learning and conventional electronic-structure calculators (ASE, PLUMED, Gaussian).
Main Results:
- SURFXsim enables finite-temperature sampling and free-energy landscape reconstruction for spin-forbidden events.
- Demonstrated applicability to distortion-driven and reactivity-driven spin-crossing.
- Compatibility with both machine-learning potentials and quantum-mechanical calculations confirmed.
Conclusions:
- SURFXsim offers a practical and flexible approach to studying spin-forbidden chemistry.
- The method extends the scale of accessible systems beyond traditional nonadiabatic dynamics.
- Facilitates deeper understanding of spin-forbidden processes in various chemical fields.
Related Concept Videos
01:08Spin–Spin Coupling Constant: Overview
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must have a...
01:20Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)
Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
01:17Spin–Spin Coupling: One-Bond Coupling
Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
03:25Free Energy Changes for Nonstandard States
The free energy change for a process taking place with reactants and products present under nonstandard conditions (pressures other than 1 bar; concentrations other than 1 M) is related to the standard free energy change according to this equation:
01:22Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the involved orbitals. The...
01:03Atomic Nuclei: Nuclear Spin State Overview
NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of one, the...