Algebraic Varieties in Second Quantization

Abstract. We develop an algebraic geometric framework for Fock space coupled cluster theory in second quantization. In quantum chemistry, many-electron states are represented as elements of the exterior algebra. The fermionic creation and annihilation operators generate the Fermi–Dirac algebra, which can be realized as a Clifford algebra acting on the exterior algebra. We present a noncommutative Gröbner basis for the Fermi–Dirac algebra, offering an alternative proof of Wick’s theorem, a fundamental result in quantum field theory. In coupled cluster theory, eigenpairs of the Schrödinger equation are approximated by a hierarchy of polynomial equations corresponding to different levels of truncation. The coupled cluster exponential parametrization of quantum states gives rise to Fock space truncation varieties. This reveals well-known varieties, such as the Grassmannian, flag varieties, and spinor varieties. We offer a detailed study of the truncation varieties, providing an explicit description of their defining equations and dimension. Furthermore, we classify all cases in which their coupled cluster degree coincides with the degree of the graph of the exponential parametrization—most notably for singleton truncations such as CCD and for the Schubert-like truncation varieties such as the Grassmannian, flag variety, and the spinor variety.