Complete O(3) Interactions from Wigner-6j Recoupling to Local O(2) Frames

Abstract

Equivariant atomistic models commonly use either Clebsch Gordan tensor product(CGTP) or edge aligned SO(2) operationsCGTP support general O(3) representations but become expensive at high angular degree, while existing local SO(2) frame methods are designed for SO(3) or the natural-parity subset of O(3). They therefore do not provide a complete local representation when polar tensors and pseudotensors occur at the same time. In this work, we introduce a generalized Wigner-6j convolution for non-collinear magnetism, electric fields, and magnetic fields, providing a general convolution scheme under O(3) that can incorporate arbitrary node-wise equivariant features. We further develop a complete O(3) convolution framework based on local O(2) frames, reducing the computational complexity of CGTPs from O(L6) to O(L3). We establish the correspondence between global O(3) irreducible representations (irreps) and local O(2) irreps and use this correspondence to construct a closed operator system comprising O2Linear, O2TensorProduct, and O2Gate.