The Hamiltonian matrix elements for single excitations, on the other hand, depend on the occupied orbitals hence on the determinant from which the excitation takes place (details in Section 2). This determinant-dependency makes the precalculation of single excitation probabilities harder, and single excitations have usually been sampled uniformly. In order to improve upon the uniform single excitation generation, Neufeld and Thom suggested to build precalculated probabilities for single excitations relying on the reference determinant, with some modifications. This approach works best for systems featuring single-reference wave functions. In the context of variational Monte Carlo, Sabzevari and Sharma suggested contracting over all orbitals in the two-electron contribution to the single excitation Hamiltonian matrix element, and to truncate all single excitations that are below a user-defined threshold. This contraction over all orbitals is independent of the determinants and can be precomputed, but the truncation introduces an artificial bias in the excitation generation; moreover, it is still uniform for all those excitations that are above the threshold.