A Deterministic Computational Framework for Exact Orientational Averaging and Complete Irreducible SO(3)...

We present a deterministic computational framework for the exact orientational averaging and complete SO(3) decomposition of ab initio dynamic first- and second-hyperpolarizability tensors in isotropic media. The method combines analytical rotational averaging with irreducible tensor decomposition to transform molecular-frame Cartesian β and γ tensors into closed laboratory-frame expressions for nonlinear optical observables, eliminating the need for stochastic orientational sampling. For the dynamic first hyperpolarizability, the response is resolved into the J = 1a, J = 1b, J = 1ab, J = 2, and J = 3 sectors, while the dynamic second hyperpolarizability is decomposed into the L = 0, L = 1, L = 2a, L = 2b, L = 3, and L = 4 sectors, with the L = 2ab cross-invariant retained for the duplicated (L = 2) block. A key formal result is that the dynamic L = 2 contribution of γ must be treated as a matrix-valued channel operator because this irreducible representation occurs with multiplicity two; accordingly, it cannot, in general, be reduced to a single scalar invariant. The orientational averages are evaluated analytically through rank-6 and rank-8 rotational tensors, yielding a general Cartesian-to-channel workflow free of Monte Carlo noise. Benchmarking against Monte Carlo rotational sampling confirms the correctness of the analytical treatment. Application to six benzothiadiazole-based donor-acceptor chromophores illustrates how molecular asymmetry, acceptor strength, and excitation wavelength redistribute the nonlinear response across symmetry channels. The methodology is general and applicable to any molecular system for which dynamic hyperpolarizability tensors are available from electronic-structure calculations.