We demonstrated x-ray structural analysis of the pressure-induced superconductor ${\beta }^{\prime }\text{−}{\mathrm{ET}}_2{\mathrm{ICl}}_2$ under extremely high pressure conditions, where ET denotes bis(ethylenedithio)tetrathiafulvalene. This material has been known as the highest transition temperature ($T_c$) superconductor among organic superconductors ($T_c=14.2\mathrm{K}$ at 8.2 GPa). On the basis of the experimental results, ab initio models were derived using the constrained random phase approximation. We revealed that the Lifshitz transition exists behind the Mott insulator–imetal transition and found that the value of the on-site Coulomb interaction was halved to around $10\mathrm{GPa}$ compared to that at ambient pressure. This Letter clarifies the enigmatic origins of high $T_{\mathrm{c}}$ and, concurrently, provides an understanding of the impacts of structural alterations in organic materials under high pressure on their electronic properties and the superconductivity process.

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• Crystal structure
• Fermi surface
• High-pressure studies
• Pressure effects
• Molecular solids
• Mott insulators
• Organic superconductors
• Strongly correlated systems
• Ab initio calculations
• First-principles calculations
• X-ray diffraction