The variety of molecular orientations and conformations in frozen solution samples also leads to distributions resonant frequencies of the spin centres, with typical disordered EPR spectra being significantly broader than the bandwidth of available rectangular microwave pulses. As a result, the Zeeman terms in the static Hamiltonian cannot be neglected, leading to partial excitation by microwave pulses and dephasing of the magnetisation during delays. To assess the impact of spectral linewidth on the fidelity of the entangling gate, we model the EPR spectra as normal distributions centred at the pulse frequencies and with standard deviation Γ. As expected, the fidelity of the entangling gate decreases as the spectral linewidth increases, rapidly falling before plateauing at around 0.63 (Figure ). This decrease in fidelity is primarily caused by partial excitation, in particular in cases where one spin is very close to resonance but its partner spin is much further away. In this situation the on-resonance spin packet effectively experiences a SFHE experiment, and so at the time of the echo its state almost completely consists of undesired in-phase coherences. It is not possible to define the linewidth of the nitroxide spectrum within this model, however the spectrum clearly extends over 200 MHz at X-band frequencies (Figure ), and so we expect that our experimental fidelity corresponds to the infinite-bandwidth limit.