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Existing computational strategies typically provide either static descriptions of crossing regions, such as minimum-energy crossing point searches, or dynamically explicit but computationally demanding nonadiabatic simulations that are difficult to apply to medium- and large-sized systems. Here, we present SURFXsim, a computational framework for simulating spin-forbidden events by combining atomistic molecular dynamics, a configuration-dependent mixed Hamiltonian, and enhanced sampling along a spin-gap collective variable. The method should be understood as a finite-temperature sampling and interpolation strategy, rather than as a direct simulation of coherent spin\u2013orbit-coupled electronic-state dynamics. It enables smooth propagation between end-state basins while using metadynamics to drive the exploration of crossing regions and reconstruct free-energy landscapes. SURFXsim is implemented in Python on top of ASE and interfaced with PLUMED, allowing the use of both machine-learning and conventional electronic-structure calculators. We illustrate the approach with distortion-driven and reactivity-driven spin-crossing examples using machine learning potentials and further demonstrate compatibility with a quantum-mechanical backend through the Gaussian package. Together, these results position SURFXsim as a practical and flexible route for studying spin-forbidden chemistry beyond the scale routinely accessible to fully nonadiabatic dynamics.<\/jats:p>","DOI":"10.1021\/acs.jctc.6c00796","type":"journal-article","created":{"date-parts":[[2026,7,27]],"date-time":"2026-07-27T05:14:03Z","timestamp":1785129243000},"page":"7512-7522","source":"Crossref","is-referenced-by-count":0,"title":["SURFXsim: Free-Energy\nSimulations of Spin-Forbidden\nEvents with a Spin-Gap Collective Variable"],"prefix":"10.1021","volume":"22","author":[{"given":"Ajmal Rahman","family":"Mullukkandy","sequence":"first","affiliation":[{"name":"Universitat Rovira i Virgili , , Marcel li\u0301 Domingo, 1 , ,","place":["Tarragona, Spain, 43007"]},{"name":"The Barcelona Institute of Science and Technology , , Avgda. Pai\u0308sos Catalans, 16 , ,","place":["Tarragona, Spain, 43007"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4913-3571","authenticated-orcid":true,"given":"Bernd","family":"Ensing","sequence":"additional","affiliation":[{"name":"University of Amsterdam , , Science Park 904 , ,","place":["XH Amsterdam, The Netherlands, 1098"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-9581-2922","authenticated-orcid":true,"given":"Carles","family":"Bo","sequence":"additional","affiliation":[{"name":"Universitat Rovira i Virgili , , Marcel li\u0301 Domingo, 1 , ,","place":["Tarragona, Spain, 43007"]},{"name":"The Barcelona Institute of Science and Technology , , Avgda. Pai\u0308sos Catalans, 16 , ,","place":["Tarragona, Spain, 43007"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3781-3107","authenticated-orcid":true,"given":"Albert","family":"Sole\u0301-Daura","sequence":"additional","affiliation":[{"name":"Universitat Rovira i Virgili , , Marcel li\u0301 Domingo, 1 , ,","place":["Tarragona, Spain, 43007"]},{"name":"The Barcelona Institute of Science and Technology , , Avgda. Pai\u0308sos Catalans, 16 , ,","place":["Tarragona, Spain, 43007"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"316","published-online":{"date-parts":[[2026,7,25]]},"reference":[{"key":"2026081111094675000_cit1","doi-asserted-by":"publisher","first-page":"1","DOI":"10.1039\/b200675h","article-title":"Spin forbidden\nchemical reactions of transition metal\ncompounds. New ideas and new computational challenges","volume":"32","author":"Poli","year":"2003","journal-title":"Chem. Soc. Rev."},{"key":"2026081111094675000_cit2","doi-asserted-by":"publisher","first-page":"331","DOI":"10.1039\/B614390C","article-title":"Understanding\nthe kinetics of spin-forbidden chemical reactions","volume":"9","author":"Harvey","year":"2007","journal-title":"Phys. Chem. Chem. Phys."},{"key":"2026081111094675000_cit3","doi-asserted-by":"publisher","first-page":"1","DOI":"10.1002\/wcms.1154","article-title":"Spin-forbidden\nreactions: computational insight into mechanisms and kinetics","volume":"4","author":"Harvey","year":"2014","journal-title":"WIREs Comput. Mol. Sci."},{"key":"2026081111094675000_cit4","doi-asserted-by":"publisher","first-page":"187","DOI":"10.1002\/wcms.83","article-title":"Spin\u2013orbit\ncoupling and intersystem crossing in molecules","volume":"2","author":"Marian","year":"2012","journal-title":"WIREs Comput. Mol. 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Chem.\nPhys."},{"key":"2026081111094675000_cit30","doi-asserted-by":"publisher","first-page":"9841","DOI":"10.1021\/j100177a043","article-title":"Reaction Kinetics of\nCoordinatively Unsaturated Iron Carbonyls Formed\non Gas-Phase Excimer Laser Photolysis of Fe(CO)5","volume":"95","author":"Ryther","year":"1991","journal-title":"J. Phys. Chem."},{"key":"2026081111094675000_cit31","doi-asserted-by":"publisher","first-page":"129","DOI":"10.1039\/b211871h","article-title":"Modelling Spin-Forbidden Reactions: Recombination of Carbon Monoxide\nwith Iron Tetracarbonyl","volume":"124","author":"Harvey","year":"2003","journal-title":"Faraday Discuss."},{"key":"2026081111094675000_cit32","doi-asserted-by":"publisher","first-page":"1123","DOI":"10.1080\/00268970500417135","article-title":"A Theoretical Study of the Spin-Forbidden Reaction\nFe(CO)4 + CO \u2192 Fe(CO)5","volume":"104","author":"Tsuchiya","year":"2006","journal-title":"Mol. 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