The recent discovery of superconductivity in ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ at a critical temperature above 80 K points to a nonconventional pairing mechanism in nickelates as in cuprates, possibly due to electronic correlations. We have calculated from first principles the electronic structure of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ under the effect of pressure and epitaxial strain including correlations by the $GW$ approximation to the many-body self-energy. We find that the Fermi surface is composed of a characteristic cuprate-shape sheet $\beta$ plus a nickelate-specific cylinder $\alpha$, both from Ni $e_g$ orbitals, with a nonnegligible drop in the quasiparticle weight and an effective 1D character. This topology results from a delicate balance between the Ni-$3d_{z^2}$ hole pocket $\gamma$, which is suppressed by correlations, and an emerging La-$5d_{x^2-y^2}$ electron pocket induced by both correlation and pressure/strain effects and whose role at low energy has been neglected so far. Unlike cuprates, the electronic structure of ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$ is already correctly described from ab initio and in agreement with the experiment without the need to introduce adjustable parameters.

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• Electronic structure
• Fermi surface
• Quasiparticles & collective excitations
• Nickelates
• Unconventional superconductors
• GW method